Ultrasonic densitometer with pre-inflated fluid coupling membranes

ABSTRACT

A contained water ultrasonic densitometer employs pre-inflated bladders holding coupling fluid about ultrasonic transducers. The bladders in a relaxed state define a cavity smaller than the human heel so that when the heel is inserted, the bladders conform to the heel in a wiping action removing entrapped air.

[0001] This application claims the benefit of provisional applicationSer. No. 60/080,158 filed Mar. 31, 1998 and is a continuation-in-part ofU.S. patent application Ser. No. 09/094,073 filed Jun. 9, 1998 which isa continuation of U.S. patent application Ser. No. 08/466,494 filed Jun.6, 1995, which is a continuation-in-part of U.S. patent application Ser.No. 08/397,027 filed Mar. 1, 1995, now U.S. Pat. No. 5,483,965 which isa continuation of U.S. patent application Ser. No. 08/72,799 filed Jun.4, 1993, abandoned, which is a continuation-in-part of U.S. patentapplication Ser. No. 07/895,494 filed Jun. 8, 1992, now U.S. Pat. No.5,343,863 which is a continuation-in-part of U.S. patent applicationSer. No. 07/772,982 filed Oct. 7, 1991, now U.S. Pat. No. 5,119,820,which is a continuation of U.S. patent application Ser. No. 07/343,170filed Apr. 25, 1989 now U.S. Pat. No. 5,054,490, which is acontinuation-in-part of Ser. No. 07/193,295 filed May 11, 1988, now U.S.Pat. No. 4,930,511.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to ultrasonic densitometerequipment using ultrasonic sound waves to measure bone integrity, and inparticular, to an ultrasonic densitometer in which the ultrasound isconducted to a human heel through liquid filled bladders.

[0004] 2. Description of the Prior Art

[0005] Various devices presently exist which may be used to measure thephysical properties and integrity of a member such as a bone.Non-invasive density measuring devices can be used to determinecumulative internal damage caused by micro-crushing and micro-fracturingoccurring in the bones of humans or animals such as race horses.Additionally, osteoporosis, or loss of bone mineralization, detection inhumans and its cure or prevention are increasingly becoming areas ofintense medical and biological interest. As the average age of the humanpopulation increases, a greater number of patients are developingcomplications due to rapid trabecular bone loss.

Early Work

[0006] U.S. Pat. No. 3,847,141 to Hoop discloses a device for measuringthe density of a bone structure, such as a finger bone or heel bone, tomonitor the calcium content thereof. The device includes a pair ofopposed spaced ultrasonic transducers which are held within a clampingdevice clamped on the bone being analyzed. A pulse generator is coupledto one of the transducers to generate an ultrasonic sound wave which isdirected through the bone to the other transducer. An electric circuitcouples the signals from the receive transducer back to the pulsegenerator for retriggering the pulse generator in response to thosesignals. The pulses therefore are produced at a frequency proportionalto the transit time that the ultrasonic wave takes to travel through thebone structure which is directly proportional to the speed of the soundthrough the bone. The speed of sound through a bone has been found to beproportional to the density of the bone. Thus the frequency at which thepulse generator is retriggered is proportional to the density of thebone.

[0007] Another device and method for establishing, in vivo the strengthof a bone is disclosed in U.S. Pat. Nos. 4,361,154 and 4,421,119 toPratt, Jr. The device includes a launching transducer and a receivingtransducer which are connected by a graduated vernier and whichdetermine the speed of sound through the bone to determine its strength.The vernier is used to measure the total transit distance between thesurfaces of the two transducers.

[0008] Lees (Lees, S. (1986) Sonic Properties of Mineralized Tissue,Tissue Characterization With Ultrasound, CRC publication 2, pp. 207-226)discusses various studies involving attenuation and speed of soundmeasurements in both cortical and spongy (cancellous or trabecular)bone. The results of these studies reveal a linear relationship betweenthe wet sonic velocity and wet cortical density, and between the drysonic velocity and the dry cortical density. The transit times of anacoustic signal through a bone member therefore are proportional to thebone density. Langton, et al. (Langton, C. M., Palmer, S. D., andPorter, S. W., (1984), The Measurement of Broad Band UltrasonicAttenuation in Cancellous Bone, Eng. Med., 13, 89-91) published theresults of a study of ultrasonic attenuation versus frequency in the oscalcis (heel bone) that is utilized through transmission techniques.These authors suggested that attenuation differences observed indifferent subjects were due to changes in the mineral content of the oscalcis. They also suggested that low frequency ultrasonic attenuationmay be a parameter useful in the diagnosis of osteoporosis or as apredictor of possible fracture risk.

Current Devices

[0009] A common site for the ultrasonic measurement is the os calcis ofthe human heel. In one ultrasonic densitometer design for measuring thissite, opposed ultrasonic transducers are placed on opposite sides of areceptacle sized to hold the human foot. The receptacle is filled withwater of a controlled temperature which serves to couple the ultrasonicenergy from a first transducer, through the gap between that transducerand the human heel, and from an exit point of the human heel backthrough a corresponding gap between the exit point and the receivingtransducer. To the extent that the water approximates the acousticimpedance of the soft tissue surrounding the heel, coupling ofultrasonic energy through the heel is improved and the precision of themeasurement increased.

[0010] While a water filled receptacle provides a simple mechanism forcoupling ultrasonic energy to the heel, it may be desired to contain thewater behind a flexible membrane or the like so as to reduce thepossibility of spilling or contaminating the water. It is known tocontain water within flexible bladders and to mount those bladders on anadjusting mechanism so that the bladders may be moved to compress theheel between them. This movement of the bladder supports may beaugmented with limited inflation of the bladders and/or movement of thetransducers. Such compressive type bladder systems create a risk of airentrapment between the bladders and the heel such as may affect thequality of the measurements. For this reason it is known to use aconical shaped membrane that initially contacts the heel at a singlepoint and, with further compression, expands outward to reduce airentrapment. Such membranes are difficult to fabricate and do not conformwell to essentially flat opposed surfaces of the heel.

BRIEF SUMMARY OF THE INVENTION

[0011] The present invention provides a contained-water ultrasonicdensitometer using stationary pre-inflated bladders. The bladders arearranged to form a cavity between them slightly smaller than the heeland the heel is slid in between the bladders. The sliding provides awiping action that helps eliminate air trapped between the bladders andthe foot.

[0012] Specifically, the densitometer provides pre-inflated bladdersopposed along an ultrasonic propagation axis. A coupling material iscontained within the bladders and ultrasonic transducers are positionedwithin the bladders to direct ultrasonic signals through the couplingmaterial along an ultrasonic propagation axis between the transducers.The bladders are compliant so as to permit them to move apart to allowinsertion of a human heel. The shape and composition of the bladdersurfaces allow the bladders to slide over and conform with the heelwhile remaining substantially in alignment with the propagation axis.

[0013] Thus, it is one object of the invention to provide acontained-water ultrasonic densitometer having significantly decreasedair entrapment between the bladders and the foot. The sliding action ofthe foot against the compliant membranes tends to reduce and eliminateentrapped air.

[0014] The first ultrasonic transducer may be held in opposition to asecond ultrasonic transducer both with fixed separation and the firstand second bladder surfaces may be mounted to a fixed support.

[0015] Thus it is another object of the invention to provide a greatlysimplified mechanism for a contained water ultrasonic densitometereliminating the need for a sliding mechanism to engage the bladders withthe foot or repeated pumping and deflation of the bladders betweenmeasurements.

[0016] The bladder surfaces may be comprised of an elastomeric membranehaving a surface coding of ultrasonic coupling gel.

[0017] Thus it is another object of the invention to provide a surfacethat easily slides along the surface of the human heel while alsoproviding good coupling for the last interface between the bladdersurface and the human heel.

[0018] The foregoing and other objects and advantages of the inventionwill appear from the following description. In this description,reference is made to the accompanying drawings which form a part hereofand in which there is shown by way of illustration, a preferredembodiment of the invention. Such embodiment does not necessarilyrepresent the full scope of the invention, however, and reference mustbe made therefore to the claims for interpreting the scope of theinvention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0019]FIG. 1 is a perspective view of the ultrasound densitometer deviceconstructed in accordance with the present invention;

[0020]FIG. 2 is a perspective view of an acoustic coupler, two of whichare shown in FIG. 1;

[0021]FIG. 3 is a front view of a transducer face from which acousticsignals are transmitted or by which acoustic signals are received, theface of the other transducer being the mirror image thereof;

[0022]FIG. 4 is a schematic block diagram view of the circuitry of theultrasound densitometer device constructed in accordance with thepresent invention;

[0023]FIG. 5 illustrates the method of sampling a received waveform usedby the circuit of FIG. 4;

[0024]FIG. 6 is a schematic block diagram view of the circuitry of analternative embodiment of an ultrasound densitometer constructed inaccordance with the present invention;

[0025]FIG. 7 is a sample of an actual ultrasonic pulse and response froman ultrasonic densitometer according to the present invention;

[0026]FIG. 8 is a sample plot of relative ultrasound pulse intensityover frequency range;

[0027]FIG. 9 is a graph in frequency domain illustrating the shift inattenuation versus frequency characteristic of a measured object ascompared to a reference;

[0028]FIG. 10 is a perspective view of an alternative embodiment of thepresent invention showing a basin for receiving a patient's foot andhaving integral opposed ultrasonic transducers;

[0029]FIG. 11 is a plan view of a foot plate and toe peg used with theembodiment of FIG. 10;

[0030]FIG. 12 is a cross-sectional detail of the foot plate of FIG. 11showing the method of attaching the sliding toe peg of the foot plate;

[0031]FIG. 13 is a block diagram of a system for transporting theacoustic coupling liquid used in the embodiment of FIG. 10;

[0032]FIG. 14 is a schematic block diagram view of the circuitry of theembodiment of FIG. 10;

[0033]FIG. 15 is an exploded view of the underside of the foot basin ofFIG. 10 showing a c-clamp for holding the opposed ultrasonic transducersin precise alignment and separation;

[0034]FIG. 16 is a perspective detailed view of the shank of the c-clampof FIG. 15 showing a lever for moving the separation of the transducersbetween an open and precisely separated closed position;

[0035]FIG. 17 is a cross-section of a human heel and ultrasonictransducers of the basin of FIG. 10 showing flexible liquid filledbladders surrounding the transducers and providing a coupling pathbetween the transducers and the heel;

[0036]FIG. 18 is a plot of the inverse of time of flight (TOF) for twobone conditions and broadband ultrasonic attenuation (BUA) as a functionof heel width showing their opposite functional dependencies;

[0037]FIG. 19 is a plot of bone quality versus bone width as might beobtained from empirical measurement of multiple bone phantoms and as maybe used to eliminate bone width effects in the ultrasonic assessment ofbone quality;

[0038]FIG. 20 is an exploded view of the elements of an ultrasonicdetector array showing a driving mechanism for improving the resolutionof the acquired data and the location of a piezoelectric film detectorarray above a spatially offset connector;

[0039]FIG. 22a is a detailed perspective fragmentary view of thepiezoelectric film detector with electrodes on its surface ascommunicating with connector terminals via acoustically transparentconductors;

[0040]FIG. 22b is a detailed fragmentary view of the piezoelectric filmof FIG. 22a showing a method of assembling the acoustically transparentconductors;

[0041]FIG. 23 is a detailed view of the face of the detector showing itsdisplacement by the driving mechanism of FIG. 20;

[0042]FIG. 24 is a figure similar to that of FIG. 17 showing use of thedetector array to provide focused reception at a point within apatient's heel;

[0043]FIG. 25 is a perspective view in phantom of a patient's heelshowing a raster scan pattern of a reception point within the heel tomeasure volumetric bone density variations within a inner and outerportion of the os calcis;

[0044]FIG. 26 is a schematic representation of a data cube collected inthe scanning shown in FIG. 25 with isodensity lines used to locate ameasurement region of interest;

[0045]FIG. 27 is a flow chart of the operation of the present inventionin locating a region of interest uniformly over several patient visits;

[0046]FIG. 28 is a perspective view of an embodiment of the inventionusing a fixed focus transducer array mechanically scanned to provide aplurality of spatially separated measurements;

[0047]FIG. 29 is a partial cross-sectional view of an alternativeembodiment to FIG. 17 using overlapping inflatable bladders;

[0048]FIG. 30 is a figure similar to FIG. 17 showing the bladdersinflated in a first mode using a first coupling liquid;

[0049]FIG. 31 is a figure similar to that of FIG. 30 showing thebladders inflated in a second mode using a second coupling liquid in theinner bladder.

[0050]FIG. 32 is a perspective exploded view and partial cutaway of anouter bladder element adapted for disposable use;

[0051]FIG. 33 is a flowchart describing a program executed with amicroprocessor 38 of FIG. 4 using the bladder configuration of FIG. 29;and

[0052]FIG. 34 is a graph representing variations in value ofattenuation, speed of sound and a combined stiffness quantity as afunction of time.

[0053]FIG. 35 is a simplified cross-sectional view of a flexiblemembrane according to the present invention showing a hemisphericaldistention for improved patient immobilization;

[0054]FIG. 36 is geometric diagram showing representational stretchingof membranes with cross axis patient movement for membranes of twodifferent aspects;

[0055]FIG. 37 is a cross sectional view of a closed system fordistending two membranes using a rolling diaphragm pump element;

[0056]FIG. 38 is a perspective view of a contained-water ultrasonicdensitometer system of the present invention showing a receptacle forreceiving a human foot holding opposed pre-inflated bladder surfaces;

[0057]FIG. 39 is a cross-sectional view along line 39-39 of FIG. 38showing configuration of the bladders within the receptacle prior toinsertion of a human foot;

[0058]FIG. 40 is a figure similar to that of FIG. 39 showing deformationof the bladders and engagement of the heel against a stop plate uponinsertion of the heel within the receptacle of the ultrasonicdensitometer;

[0059]FIG. 41 is a cross-sectional view taken along lines 41-41 of FIG.38 showing seating of the foot against the stop plate of FIG. 40 andmeasurement of the forces so produced to ensure proper seating of thefoot; and

[0060]FIG. 42 is a chart showing downward and backward forces impartedby the foot on the stop plate and a defined region of proper forceenabling a measurement by the densitometer.

DETAILED DESCRIPTION OF THE INVENTION Caliper Embodiment

[0061] Referring more particularly to the drawings, wherein like numbersrefer to like parts, FIG. 1 shows a portable ultrasound densitometer 10for measuring the physical properties and integrity of a member, such asa bone, in vivo. The densitometer 10 as shown in FIG. 1 includes ahandle 11 with actuator button 12. Extending linearly from the handle 11is a connection rod 13. The densitometer 10 also includes a fixed arm 15and an adjustable arm 16. The fixed arm 15 preferably is formedcontinuously with the connection rod 13, and therefore is connected toan end 17 of the connection rod 13. The adjustable arm 16 is slidablymounted on the connection rod 13 between the handle 11 and a digitaldisplay 18 mounted on the rod 13. The knob 19 may be turned so as to belocked or unlocked to allow the adjustable arm 16 to be slid along theconnection rod 13 so that the distance between the arms 15 and 16 may beadjusted.

[0062] Connected at the end of the fixed arm 15 is a first (left)transducer 21 and at the end of the adjustable arm 16 is a second(right) transducer 21. As shown in FIGS. I and 2, each of thetransducers 21 has mounted on it a respective compliant acoustic coupler23 to acoustically couple the transducer to the object being tested. Theacoustic coupler 23 includes a plastic ring 24 and attached pad 26formed of urethane or other compliant material. FIG. 3 shows a face 28of the first (left) transducer 21 which is normally hidden behind thecompliant pad 26 of the acoustic coupler 23. The transducer face 28normally abuts against the inner surface 29 of the pad 26 shown in FIG.2. The transducer face 28 shown in FIG. 3 includes an array of twelvetransducer elements labeled a-l. The second (right) transducer 21includes a face 28 which is the mirror image of that shown in FIG. 3.

[0063]FIG. 4 generally shows in schematic fashion the electroniccircuitry 31 of the densitometer 10, which is physically contained inthe housing of the digital display 18. An object 32 is placed betweenthe two transducers 21 so that acoustic signals may be transmittedthrough the object. This object 32 represents a member, such as a bone,or some material with known acoustic properties such as distilled wateror a neoprene reference block. As shown in the embodiment illustrated inFIG. 4, the leftmost transducer 21 is a transmit transducer and therightmost transducer 21 a receive transducer. In fact though, either orboth of the transducers 21 may be a transmit and/or receive transducer.The transmit and receive transducers 21 of the circuit of FIG. 4 areconnected by element select signals 36 and 37 to a microprocessor 38.The microprocessor 38 is programmed to determine which one of therespective pairs of transducer elements a through l are to betransmitting and receiving at any one time. This selection isaccomplished by the element select signal lines 36 and 37, which may beeither multiple signal lines or a serial data line to transmit theneeded selection data to the transducers 21. The microprocessor 38 isalso connected by a data and address bus 40 to the digital display 18, adigital signal processor 41, a sampling analog to digital converter 42,and a set of external timers 43. The microprocessor 38 has “on board”electrically programmable non-volatile random access memory (NVRAM) and,perhaps as well, conventional RAM memory, and controls the operations ofthe densitometer 10. The digital signal processor 41 has “on board”read-only memory (ROM) and performs many of the mathematical functionscarried out by the densitometer 10 under the control of themicroprocessor 38. The digital signal processor 41 specifically includesthe capability to perform discrete Fourier transforms, as iscommercially available in integrated circuit form presently, so as to beable to convert received waveform signals from the time domain to thefrequency domain. The microprocessor 38 and digital signal processor 41are interconnected also by the control signals 45 and 46 so that themicroprocessor 38 can maintain control over the operations of thedigital signal processor 41 and receive status information back.Together the microprocessor 38 and the digital signal processor 41control the electrical circuit 31 so that the densitometer 10 can carryout its operations, which will be discussed below. An auditory feedbackmechanism 48, such as an audio speaker, can be connected to themicroprocessor 38 through an output signal 49.

[0064] The external timer 43 provides a series of clock signals 51 and52 to the A/D converter 42 to provide time information to the A/Dconverter 42 so that it will sample at timed intervals electricalsignals which it receives ultimately from the transmit transducer, inaccordance with the program in the microprocessor 38 and the digitalsignal processor 41. The external timer 43 also creates a clock signal53 connected to an excitation amplifier 55 with digitally controllablegain. Timed pulses are generated by the timer 43 and sent through thesignal line 53 to the amplifier 55 to be amplified and directed to thetransmit transducer 21 through the signal line 56. The transmittransducer 21 converts the amplified pulse into an acoustic signal whichis transmitted through the object or material 32 to be received by thereceive transducer 21 which converts the acoustic signal back to anelectrical signal. The electrical signal is directed through outputsignal 57 to a receiver amplifier 59 which amplifies the electricalsignal.

[0065] The excitation amplifier circuit 55 is preferably a digitallycontrollable circuit designed to create a pulsed output. Theamplification of the pulse can be digitally controlled in steps from oneto ninety-nine. In this way, the pulse can be repetitively increased inamplitude under digital control until a received pulse of appropriateamplitude is received at the receiver/amplifier circuit 59, where thegain is also digitally adjustable.

[0066] Connected to the receiver amplifier circuit 59 and integraltherewith is a digitally controllable automatic gain control circuitwhich optimizes the sensitivity of the receive transducer 21 and theamplifier circuit 59 to received acoustic signals. The microprocessor 38is connected to the amplifier circuit and automatic gain control 59through signal line 60 to regulate the amplification of the amplifiercircuit and gain control 59. The amplified electric signals are directedthrough lead 61 to the A/D converter 42 which samples those signals attimed intervals. The A/D converter 42 therefore in effect samples thereceived acoustic signals. As a series of substantially identicalacoustic signals are received by the receive transducer 21, the A/Dconverter 42 progressively samples an incremental portion of eachsuccessive signal waveform. The microprocessor 38 is programmed so thatthose portions are combined to form a digital composite waveform whichis nearly identical to a single waveform. This digitized waveform may bedisplayed on the digital display 18, or processed for numerical analysisby the digital signal processor 41.

[0067] The densitometer constructed in accordance with FIGS. 1-4 can beoperated in one or more of several distinct methods to measure thephysical properties of the member, such as integrity or density. Thedifferent methods, as described in further detail below, depend both onthe software programming the operation of the microprocessor 34 as wellas the instructions given to the clinician as to how to use thedensitometer. The different methods of use may all be programmed into asingle unit, in which case a user-selectable switch may be provided toselect the mode of operation, or a given densitometer could beconstructed to be dedicated to a single mode of use. In any event, forthe method of use of the densitometer to measure the physical propertiesof a member to be fully understood, it is first necessary to understandthe internal operation of the densitometer itself.

[0068] In any of its methods of use, the densitometer is intended to beplaced at some point in the process on the member whose properties arebeing measured. This is done by placing the transducers 21 on theopposite sides of the member. To accomplish this, the knob 19 isloosened to allow the adjustable arm 16 to be moved so that thetransducers 21 can be placed on opposite sides of the member, such asthe heel of a human patient. The outside surfaces of the pads 26 can beplaced against the heel of the subject with an ultrasound gel 35 orother coupling material placed between the pads 26 and subject 32 toallow for improved transmission of the acoustic signals between themember 32 and transducers 21. Once the transducers 21 are properlyplaced on the member, the knob 19 may be tightened to hold theadjustable arm 16 in place, with the transducers 21 in spaced relationto each other with the member 32 therebetween. The actuator button 12may then be pressed so that acoustic signals will be transmitted throughthe member 32 to be received by the receive transducer 21. Theelectronic circuit of FIG. 4 receives the electrical signals from thereceive transducer 21, and samples and processes these signals to obtaininformation on the physical properties and integrity of the member 32 invivo. The microprocessor 38 is programmed to indicate on the digitaldisplay 18 when this information gathering process is complete.Alternatively, the information may be displayed on the digital display18 when the information gathering process is completed. For example, thetransit time of the acoustic signals through the member 32 could simplybe displayed on the digital display 18.

[0069] Considering in detail the operation of the circuitry of FIG. 4,the general concept is that the circuitry is designed to create anultrasonic pulse which travels from transmit transducer 21 through thesubject 32 and is then received by the receive transducer 21. Thecircuitry is designed to both determine the transit time of the pulsethrough the member 32 (also called time of flight (“TOF”)), to ascertainthe attenuation of the pulse through the member 32, and to be able toreconstruct a digital representation of the waveform of the pulse afterit has passed through the member 32, so that it may be analyzed todetermine the attenuation at selected frequencies. To accomplish all ofthese objectives, the circuitry of FIG. 4 operates under the control ofthe microprocessor 38. The microprocessor 38 selectively selects,through the element select signal lines 36, a corresponding pair or agroup of the elements a through I on the face of each of the transducers21. The corresponding elements on each transducer are selectedsimultaneously while the remaining elements on the face of eachtransducer are inactive. With a given element, say for example element aselected, the microprocessor then causes the external timer 43 to emit apulse on signal line 53 to the excitation amplifier circuit 55. Theoutput of the excitation amplifier 55 travels along signal line 56 toelement a of the transmit transducer 21, which thereupon emits theultrasonic pulse. The corresponding element a on the receive transducer21 receives the pulse and presents its output on the signal line 57 tothe amplifier circuit 59. What is desired as an output of the A/Dconverter 42 is a digital representation of the analog waveform which isthe output of the single transducer element which has been selected.Unfortunately, “real time” sampling A/D converters which can operaterapidly enough to sample a waveform at ultrasonic frequencies arerelatively expensive. Therefore it is preferred that the A/D converter42 be an “equivalent time” sampling A/D converter. By “equivalent time”sampling, it is meant that the A/D converter 42 samples the output ofthe transducer during a narrow time period after any given ultrasonicpulse. The general concept is illustrated in FIG. 5. The typicalwaveform of a single pulse received by the receive transducer 21 andimposed on the signal line 57 is indicated by a function “f”. The samepulse is repetitively received as an excitation pulse and isrepetitively launched. The received pulse is sampled at a sequence oftime periods labeled t₀-t₁₀. In other words, rather than trying to do areal-time analog to digital conversion of the signal f, the signal issampled during individual fixed time periods t₀-t₁₀ after the transmitpulse is imposed, the analog value during each time period is convertedto a digital function, and that data is stored. Thus the total analogwaveform response can be recreated from the individual digital valuescreated during each time period t, with the overall fidelity of therecreation of the waveform dependent on the number of time periods twhich are sampled. The sampling is not accomplished during a single realtime pulse from the receive transducer 21. Instead, a series of pulsesare emitted from the transmit transducer 21. The external timer isconstructed to provide signals to the sampling A/D converter 42 alongsignal lines 51 and 52 such that the analog value sampled at time periodt₀ when the first pulse is applied to a given transducer element, thenat time t₁ during the second pulse, time t₂ during the third pulse, etc.until all the time periods are sampled. Only after the complete waveformhas been sampled for each element is the next element, i.e. element b,selected. The output from the A/D converter 42 is provided both to themicroprocessor 38 and to the signal processor 41. Thus the digitaloutput values representing the complex waveform f of FIG. 5 can beprocessed by the signal processor 41 after they are compiled for eachtransducer element. The waveform can then be analyzed for time delay orattenuation for any given frequency component with respect to thecharacteristic of the transmitted ultrasonic pulse. The process is thenrepeated for the other elements until all elements have been utilized totransmit a series of pulses sufficient to create digital datarepresenting the waveform which was received at the receive transducerarray 21. It is this data which may then be utilized in a variety ofmethods for determining the physical properties of the member. Dependingon the manner in which the densitometer is being utilized and the databeing sought, the appropriate output can be provided from either themicroprocessor 38 or the signal processor 41 through the digital display18.

[0070] Because the ultrasonic pulsing and sampling can be performed sorapidly, at least in human terms, the process of creating a sampledultrasonic received pulse can optionally be repeated several times toreduce noise by signal averaging. If this option is to be implemented,the process of repetitively launching ultrasonic pulses and sampling thereceived waveform as illustrated in FIG. 5 is repeated one or more timesfor each element in the array before proceeding to the next element.Then the sampled waveforms thus produced can be digitally averaged toproduce a composite waveform that will have a lesser random noisecomponent than any single sampled waveform. The number of repetitionsnecessary to sufficiently reduce noise can be determined by testing in afashion known to one skilled in the art.

[0071] Having thus reviewed the internal operation of the densitometerof FIGS. 1-4, it is now possible to understand the methods of use of thedensitometer to measure the physical properties of the member. The firstmethod of use involves measuring transit time of an ultrasonic pulsethrough a subject and comparing that time to the, time an ultrasonicpulse requires to travel an equal distance in a substance of knownacoustic properties such as water. To use the densitometer in thisprocedure, the adjustable arm 16 is adjusted until the member of thesubject, such as the heel, is clamped between the transducers 21. Thenthe knob 19 is tightened to fix the adjustable arm in place. Theactuator button 12 is then pressed to initiate a pulse and measurement.Next the densitometer is removed from the subject while keeping the knob19 tight so that the distance between the transducers 21 remains thesame. The device 10 is then placed about or immersed in a standardmaterial 32 with known acoustic properties, such as by immersion in abath of distilled water. The actuator button 12 is pressed again so thatacoustic signals are transmitted from the transmit transducer 21 throughthe material 32 to the receive transducer 21. While it is advantageousto utilize the whole array of elements a through l for the measurementof the member, it may only be necessary to use a single pair of elementsfor the measurement through the standard assuming only that the standardis homogeneous, unlike the member. The signal profiles received by thetwo measurements are then analyzed by the microprocessor 38 and thesignal processor 41. This analysis can be directed both to thecomparative time of transit of the pulse through the subject as comparedto the standard and to the characteristics of the waveform in frequencyresponse and attenuation through the subject as compared to thestandard.

[0072] Thus in this method the densitometer may determine the physicalproperties and integrity of the member 32 by both or either of two formsof analysis. The densitometer may compare the transit time of theacoustic signals through the member with the transmit time of theacoustic signals through the material of known acoustic properties,and/or the device 10 may compare the attenuation as a function offrequency of the broadband acoustic signals through the member 32 withthe attenuation of corresponding specific frequency components of theacoustic signals through the material of known acoustic properties. The“attenuation” of an acoustic signal through a substance is thediminution of the ultrasonic waveform from the propagation througheither the subject or the standard. The theory and experiments usingboth of these methods are presented and discussed in Rossman, P. J.,Measurements of Ultrasonic Velocity and Attenuation In The Human OsCalcis and Their Relationships to Photon Absorptiometry Bone MineralMeasurements (1987) (a thesis submitted in partial fulfillment of therequirements for the degree of Master of Science at the University ofWisconsin-Madison). Tests have indicated that there exists a linearrelationship between ultrasonic attenuation (measured in decibels) (dB))at specific frequencies, and those frequencies. The slope (dB/MHz) ofthe linear relationship, referred to as the broadband ultrasonicattenuation, is dependent upon the physical properties and integrity ofthe substance being tested. With a bone, the slope of the linearrelationship would be dependent upon the bone mineral density. Thusbroadband ultrasonic attenuation through a bone is a parameter directlyrelated to the quality of the cancellous bone matrix.

[0073] The microprocessor 38 may therefore be programmed so that thedevice determines the physical properties and integrity of the member bycomparing either relative transit times and/or relative broadbandultrasonic attenuation through the member and a material of knownacoustic properties. When comparing the transit times, themicroprocessor 38 may be programmed most simply so that the electronics,having received the acoustic signals after they have been transmittedthrough the member, determines the “member” transit time of thoseacoustic signals through the member, and after the acoustic signals havebeen transmitted through the material of known acoustic properties,determines the “material” transit time of the acoustic signals throughthe material. These time periods may be measured most simply by countingthe number of clock pulses of known frequency emitted by the timer 43between the time of launching the pulse and the sensing of the receivedpulse at the A/D converter 42. The microprocessor 38 then makes amathematical “time” comparison of the member transit time to thematerial transit time and then relates that mathematical time comparisonto the physical properties and integrity of the member. The mathematicaltime comparison may be made by either determining a difference betweenthe member transit time and the material transit time, or by determininga ratio between the member transit time and the material transit time.

[0074] As a second method of using the densitometer, it may alsodetermine the physical properties and integrity of the member 32 bydetermining and comparing the attenuation of the broadband frequencycomponents of the acoustic signals through the member without referenceto a material having known acoustic properties. Using this method, thecomparison of velocity to a standard is not necessary and absolutetransit time of the pulse need not be calculated since it is attenuationthat is measured. In such a mode, it is preferable that the transmittransducer 21 transmits an acoustic signal which has a broad range offrequency components, such as a simple ultrasonic pulse. In any case,the acoustic signal should have at least one specific frequencycomponent.

[0075] In this attenuation comparison mode, the microprocessor 38 isprogrammed so that after the receive transducer 21 receives the acousticsignals transmitted through the bone member 32, it determines theabsolute attenuation through the member 32 of the frequency componentspectrum of the acoustic signals. It is to facilitate the measurement ofattenuation that the excitation amplifier circuit 55 and the receiveramplifier 59 have amplification levels which may be digitallycontrolled. By successively varying the gain of the amplifiers 55 and 59on successive pulses, the circuit of FIG. 4 can determine what level ofgain is necessary to place the peak of the received waveform at a propervoltage level. This gain is, of course, a function of the level ofattenuation of the acoustic pulse during transit through the member 32.After the receive transducer 21 receives acoustic signals,microprocessor 38 in conjunction with the signal processor 41 determinesthe absolute attenuation of individual specific frequency components ofthe received acoustic signal transmitted through the material. Thedigital signal processor 41 then makes mathematical “attenuation”comparisons of the corresponding individual specific frequencycomponents through the member. A set of mathematical attenuationcomparisons between corresponding frequency components may be therebyobtained, one comparison for each frequency component compared. Themanner in which the attenuation functions with respect to frequency canthus be derived. The microprocessor 38 and digital signal processor 41then relate that function to the physical properties and integrity ofthe member.

[0076] Shown in FIG. 7 is a sample broadband ultrasonic pulse and atypical received waveform. To achieve an ultrasonic signal that is verybroad in the frequency domain, i.e., a broadband transmitted signal, anelectronic pulse such as indicated at 70 is applied to the selectedultrasonic transducer in the transmit array 21 which then resonates witha broadband ultrasonic emission. The received signal, such as indicatedat 72 in FIG. 7 in a time domain signal plot, is then processed bydiscrete Fourier transform analysis so that it is converted to thefrequency domain. Shown in FIG. 8 is a pair of plots of sample receivedsignals, in frequency domain plots, showing the shift in received signalintensity as a function of frequency between a reference object and aplug of neoprene placed in the instrument. FIG. 9 illustrates a similarcomparison, with FIG. 8 using relative attenuation in the verticaldimension and FIG. 9 using power of the received signal using a similarreference material. Both representations illustrate the difference inrelative intensities as a function of frequency illustrating howbroadband ultrasonic attenuation varies from object to object. Theactual value calculated, broadband ultrasonic attenuation, is calculatedby first comparing the received signal against the reference signal,then performing the discrete Fourier transform to convert to frequencydomain, then performing a linear regression of the difference inattenuation slope to derive broadband ultrasonic attenuation.

[0077] The mathematics of the discrete Fourier transform are such thatanother parameter related to bone member density may be calculated inaddition to, or in substitution for, broadband attenuation (sometimesreferred to as “attenuation” or “BUA” below). When the discrete Fouriertransform is performed on the time-domain signal, the solution for eachpoint includes a real member component and an imaginary membercomponent. The values graphed in FIGS. 8 and 9 are the amplitude of thereceived pulse as determined from this discrete Fourier transform bytaking the square root of the sum of the squares of the real componentand the imaginary component. The phase angle of the change in phase ofthe ultrasonic pulse as it passed through the member can be calculatedby taking the arctangent of the ratio of the imaginary to the realcomponents. This phase angle value is also calculated to bone memberdensity.

[0078] The microprocessor 38 may also be programmed so that thedensitometer simultaneously performs both functions, i.e. determinesboth transit time and absolute attenuation of the transmitted acousticsignals, first through the member and then through the material withknown acoustic properties. The densitometer may then both derive thebroadband ultrasonic attenuation function and make a mathematical timecomparison of the member transit time to the material transit time. Themicroprocessor 38 and digital signal processor 41 then relate both thetime comparison along with the attenuation function to the physicalproperties and integrity, or density of the member 32.

[0079] In yet another possible mode of operation, the microprocessor 38may be programmed so that the densitometer 10 operates in a mode wherebythe need for calculating either the relative transit time or theattenuation of the acoustic signals through a material of known acousticproperties is eliminated. In order to operate in such a mode, themicroprocessor 38 would include a database of normal absolute transittimes which are based upon such factors as the age, height, weight, raceor the sex of the individual being tested as well as the distancebetween the transducers or the thickness or size of the member. Thisdatabase of normal transit times can be stored in the non-volatilememory or could be stored in other media. When testing an individual inthis mode, the relevant factors for the individual are placed into themicroprocessor 38 to select the pertinent normal transit time based onthose factors. The transducers 21 are placed on the bone member beingtested as described above. When the actuator button 12 is pressed, theacoustic signals are transmitted through the member 32. The receivetransducer 21 receives those signals after they have been transmittedthrough the member, and the electronics 31 then determine the “member”transit time of the acoustic signals through the member. Themicroprocessor 38 and digital signal processor 41 then make amathematical comparison of the measured member transit time to theselected database normal transit time, and relate the mathematical timecomparison to the physical properties and integrity, or density of themember, which is displayed.

[0080] As an alternative output of the densitometer of the presentinvention, the digital display 18 could also include a displaycorresponding to the pattern of the array of elements on the face of thetransducer 21 as seen in FIG. 3. This display could then display, foreach element a through l, a gray scale image proportional to theparameter, i.e. transit time or attenuation, being measured. This imagemay provide a visual indication to an experienced clinician as to thephysical properties of the member present in the patient.

[0081] Shown in FIG. 6 is a circuit schematic for an alternativeembodiment of an ultrasonic densitometer constructed in accordance withthe present invention. In the circuit of FIG. 6, parts having similarstructure and function to their corresponding parts in FIG. 4 areindicated with similar reference numerals.

[0082] The embodiment of FIG. 6 is intended to function with only asingle transducer array 21 which functions both as the transmit and thereceive transducer array. An optional reflecting surface 64 may beplaced on the opposite side of the member 32 from the transducer array21. A digitally controlled multiple pole switch 66, preferably anelectronic switch rather than a mechanical one, connects the input toand output from the elements of the transducer array 21 selectivelyeither to the excitation amplifier 55 or to the controllable gainreceiver/amplifier circuit 59. The switch 66 is connected by a switchcontrol line 68 to an output of the microprocessor 38.

[0083] In the operation of the circuit of FIG. 6, it functions in mostrespects like the circuit of FIG. 4, so only the differences need bediscussed. During the launching of an ultrasonic pulse, themicroprocessor 38 causes a signal to appear on the switch control line68 to cause the switch 66 to connect the output of the excitationamplifier 55 to the selected element in the transducer array 21.Following completion of the launching of the pulse, the microprocessor38 changes the signal on the switch control line 68 to operate theswitch 66 to connect the selected element or elements as an input to theamplifier 59. Meanwhile, the pulse propagates through the member 32. Asthe pulse transits through the member, reflective pulses will begenerated as the pulse crosses interfaces of differing materials in themember and, in particular, as the pulse exits the member into the air atthe opposite side of the member. If the transition from the member toair does not produce a sufficient reflective pulse, the reflectingsurface 64 can be placed against the opposite side of the member toprovide an enhanced reflected pulse.

[0084] The embodiment of FIG. 6 can thus be used to analyze the physicalproperties and integrity of a member using only one transducer 21. Allof the methods described above for such measurements may be used equallyeffectively with this version of the device. The transit time of thepulse through the member can be measured simply by measuring the timeperiod until receipt of the reflected pulse, and then simply dividing bytwo. This time period can be compared to the transit time, over asimilar distance, through a standard medium such as water. The timeperiod for receipt of the reflected pulse could also be simply comparedto standard values for age, sex, etc. Attenuation measurements to detectdifferential frequency measurement can be directly made on the reflectedpulse. If no reflecting surface 64 is used, and it is desired todetermine absolute transit time, the thickness of the member or samplecan be measured.

[0085] The use of the multi-element ultrasonic transducer array for thetransducers 21, as illustrated in FIG. 3, enables another advantageousfeature of the instrument of FIGS. 1-9. In using prior artdensitometers, it was often necessary to precisely position theinstrument relative to the body member of the patient being measured tohave useful results. The difficulty arises because of heterogeneities inthe bone mass and structure of actual body members. A measurement takenat one location of density may be significantly different from ameasurement taken close by. Therefore prior art instruments fixed thebody member precisely so that the measurement could be taken at theprecise location each time.

[0086] The use of the ultrasonic transducer array obviates the need forthis precise positioning. Using the instrument of FIGS. 1-9, theinstrument performs a pulse and response, performs the discrete Fouriertransform, and generates a value for broadband ultrasonic attenuationfor each pair of transducer elements a through l. Then themicroprocessor 38 analyzes the resulting array of bone ultrasonicdensity measurements to reproducibly identify the same region ofinterest each time. In other words, since the physical array oftransducers is large enough to reliably cover at least the one commonregion of interest each time, the measurement is localized at the samelocus each time by electrically selecting the proper location for themeasurement from among the locations measured by the array. Theinstrument of FIGS. 1-9 is conveniently used by measuring the density ofthe os calcis as measured through the heel of a human patient. When usedin this location, it has been found that a region of interest in the oscalcis can be located reliably and repeatedly based on the comparisonsof broadband ultrasonic attenuation at the points in the array. Theregion of interest in the os calcis is identified as a local or relativeminimum in broadband ultrasonic attenuation and/or velocity closelyadjacent the region of highest attenuation values in the body member.Thus repetitive measurements of the broadband ultrasonic attenuationvalue at this same region of interest can be reproducibly taken eventhough the densitometer instrument 10 is only generally positioned atthe same location for each successive measurement.

[0087] This technique of using a multiple element array to avoidposition criticality is applicable to other techniques other than thedetermination of broadband ultrasonic attenuation as described here. Theconcept of using an array and comparing the array of results todetermine measurement locus would be equally applicable to measurementstaken of member-density based on speed of sound transit time, othermeasurements of attenuation or on the calculation of phase anglediscussed above. The use of such a multiple-element array, withautomated selection of one element in the region of interest, can alsobe applied to other measurement techniques useful for generatingparameters related to bone member density, such as measuring speedchanges in the transmitted pulse such as suggested in U.S. Pat. No.4,361,154 to Pratt, or measuring the frequency of a “sing-around”self-triggering pulse as suggested in U.S. Pat. No. 3,847,141 to Hoop.The concept which permits the position independence feature is that ofan array of measurements generating an array of data points from which aregion of interest is selected by a reproducible criterion or severalcriteria. The number of elements in the array also clearly can be variedwith a larger number of elements resulting in a greater accuracy inidentifying the same region of interest.

[0088] In this way, the ultrasound densitometer of the present inventionprovides a device capable of rapid and efficient determination of thephysical properties of a member in vivo without the use of radiation.Because the densitometer is constructed to operate under the control ofthe microprocessor 38, it can be programmed to operate in one of severalmodes, as discussed above. This allows both for flexibility to clinicalgoals as well as efficient use of the device.

Basin Embodiment

[0089] Shown in FIG. 10 is another variation on an ultrasonicdensitometer constructed in accordance with the present invention. Inthe densitometer 100 of FIG. 10, there are two ultrasonic transducerarrays 121, which are generally similar to the ultrasonic transducerarrays 21 of the embodiment of FIG. 1, except that the transducer arrays21 are fixed in position rather than movable.

[0090] The densitometer 100 includes a generally box-shaped mountingcase 101 with sloping upper face 102 in which is formed a basin 103. Thebasin 103 is sized to receive a human foot and is generally trigonousalong a vertical plane aligned with the length of the foot so that whenthe foot is placed within the basin 103, the toes of the foot areslightly elevated with respect to the heel of the foot.

[0091] The transducer arrays 121 are positioned in the case 101 so thatthey extend into the basin 103 to be on opposite sides of the heel ofthe foot placed in the basin 103. When the foot is in position withinthe basin 103, the sole of the foot may rest directly on a bottom 104 ofthe basin 103 with the heel of the foot received within a curved pocket106 forming a back wall of the basin 103. As so positioned, thetransducer arrays 121 are on either side of the os calcis. It has beendemonstrated that placing the transducer approximately 4 centimeters upfrom the sole and 3.5 centimeters forwardly from the rearward edge ofthe heel places the transducers in the desired region and focused on theos calcis.

[0092] The foot may, alternatively, rest on a generally planar footplate 108 having a contour conforming to the bottom 104 and placedagainst the bottom 104 between the foot and the bottom 104. The footplate 108 holds an upwardly extending toe peg 110 for use in reducingmotion of the foot during the measurement process. Referring to FIG. 11,the toe peg 110 is sized to fit between the big toe and the nextadjacent toe of a typical human foot and is mounted in a slot 112 so asto be adjustable generally along the length of the foot to accommodatethe particular length of the foot.

[0093] The slot 112 cants inward toward a medial axis 114 of the foot,defined along the foot's length, as one moves along the slot 112 towardsthe portion of the foot plate 108 near the heel of the foot. Thiscanting reflects the general relation between foot length and width andallows simple adjustment for both dimensions at once.

[0094] The toe peg 110 is sized to fit loosely between the toes of thefoot without discomfort and does not completely prevent voluntarymovement of the foot. Nevertheless, it has been found that the tactilefeedback to the patient provided by the toe peg 110 significantlyreduces foot movement during operation of the densitometer 100. Twodifferent foot plates 108, being mirror images of each other, are usedfor the left and right foot.

[0095] Referring to FIG. 12, the toe peg 110 is held to the slot 112 bya fastener 111 having a threaded portion which engages correspondingthreads in the toe peg 110. The head of the threaded fastener 111engages the slot 112 so as to resist rotation. Thus, the toe peg 110 maybe fixed at any position along the length of the slot 112 by simplyturning the toe peg 110 slightly about its axis to tighten the threadedfastener 111 against the foot plate 108.

[0096] Referring again to FIG. 10, the basin 103 of the densitometer 110is flanked, on the upper face 102 of the enclosure 101, by two foot restareas 116 and 118 on the left and right side respectively. Forexamination of a patient's right foot, the patient's left foot may reston foot rest area 118 while the patient's right foot may be placedwithin basin 103. Conversely, for examination of the patient's leftfoot, the left foot of the patient is placed within basin 103 and thepatient's right foot may rest on foot rest area 116. The foot rest areashave a slope conforming to that of the upper face 102 and approximatelythat of bottom 104. The flanking foot rest areas 116 and 118 allow thedensitometer 100 to be used in comfort by a seated patient.

[0097] When the densitometer 100 is not in use, the basin area 103 iscovered with a generally planar cover 120 hinged along the lower edge ofthe basin 103 to move between a closed position substantially within theplane of the upper face 102 and covering the basin 103, and an openposition with the plane of the cover 120 forming an angle a with thebottom 104 of the basin 103 as held by hinge stops 122. The angle α isapproximately 90° and selected so as to comfortably support the calf ofthe patient when the patient's foot is in place within basin 103. Tothat end, the upper surface of the cover 120, when the cover 120 is inthe open position, forms a curved trough to receive a typical calf.

[0098] The support of the patient's calf provided by the cover 120 hasbeen found to reduce foot motion during operation of the densitometer100.

[0099] Referring now to FIGS. 10 and 12, because the densitometer 100employs fixed transducers 121, a coupling liquid is provided in thebasin 103 to provide a low loss path for acoustic energy between thetransducers 121 and the patient's foot regardless of the dimensions ofthe latter. The coupling liquid is preferably water plus a surfactant,the latter which has been found to improve the signal quality andconsistency of the reading of the densitometer. The surfactant may be,for example, a commercially available detergent. It will be recognized,however, that other flowable, acoustically conductive media may be usedto provide acoustic coupling, and hence, that the term “coupling liquid”should be considered to embrace materials having a viscosity higher thanthat of water such as, for example, water based slurries and thixotropicgels.

[0100] For reasons of hygiene, the exhaustion of the surfactant, andpossible reduction of signal quality with the collection of impuritiesin the coupling liquid, it has been determined that the liquid in thebasin 103 should be changed in between each use of the densitometer 103.Changing this liquid is time consuming and ordinarily would requireconvenient access to a sink or the like, access which is not alwaysavailable. Failure to change the liquid may have no immediate visibleeffect, and hence changing the liquid is easy to forget or delay. Forthis reason, the present embodiment employs an automated liquid handlingsystem linked to the ultrasonic measurement operation through circuitrycontrolled by microprocessor 38 to be described.

[0101] Referring to FIG. 13 in the present embodiment, premixed waterand surfactant for filling the basin 103 are contained in a removablepolypropylene supply tank 124, whereas exhausted water and surfactantfrom the basin 103 are received by a similar drain tank 126. Each tank124 and 126 contains a manual valve 128 which is opened when the tanksare installed in the densitometer 100 and closed for transporting thetanks to a remote water supply or drain. The supply tank 124 and thedrain tank 126 have vents 150, at their upper edges as they are normallypositioned, to allow air to be drawn into or expelled from the interiorof the tanks 124 and 126 when they are in their normal position withinthe densitometer 100 and valves 128 are open. The tanks 124 and 126 holdsufficient water for approximately a day's use of the densitometer 100and thus eliminate the need for convenient access to plumbing.

[0102] The valve 128 of the supply tank 124 connects the tank throughflexible tubing to a pump 130 which may pump liquid from the supply tank124 to a heating chamber 132.

[0103] Referring to FIG. 14, the heating chamber 132 incorporates aresistive heating element 164 which is supplied with electrical currentthrough a thermal protection module in thermal contact with the couplingliquid in the heating chamber 132. The thermal protection module 166includes a thermostat and a thermal fuse, as will be described below. Athermistor 168, also in thermal communication with the liquid in theheating chamber, provides a measure of the liquid's temperature duringoperation of the densitometer 100. The heater chamber 132 additionallyincorporates an optical level sensor 172. The level sensor 172 detectsthe level of liquid in the heating chamber 132 by monitoring changes inthe optical properties of a prism system when the prism is immersed inliquid as opposed to being surrounded by air. The operation of thethermistor 168 and the level sensor 172 will be described further below.

[0104] Referring again to FIG. 13, the heating chamber 132 communicatesthrough an overflow port 134 and flexible tubing to an overflow drainoutlet 136. The overflow outlet 136 is positioned at the bottom of thedensitometer 100 removed from its internal electronics. The overflowport 134 is positioned above the normal fill height of the heatingchamber 132 as will be described in detail below.

[0105] The heating chamber 132 also communicates, through its lowermostpoint, with an electrically actuated fill valve 138 which provides apath, through flexible tubing, to a fill port 140 positioned in the wallof basin 103.

[0106] In the opposite wall of the basin 103 is an overflow port 142which opens into the basin 103 at a point above the normal fill heightof the basin 103 and which further communicates, through a T-connector144, to the drain tank 126.

[0107] A drain 146, in the bottom 104 of the basin 103, provides a pathto an electronically actuated drain valve 148. The drain valve 148operates to allow liquid in the basin 103 to flow through the drain 146to the T-connector 144 and into the drain tank 126. The overflow port142 and drain 146 incorporate screens 152 to prevent debris fromclogging the tubing or the drain valve 148 communicating with the draintank 126.

[0108] Referring now to FIGS. 10 and 13, the supply tank 124 and thedrain tank 126 are positioned within the case 101 of the densitometer100 and located at a height with respect to the basin 103 so that liquidwill drain from the basin 103 into the drain tank 126 solely under theinfluence of gravity and so that gravity alone is not sufficient to fillthe basin 103 from supply tank 124 when fill valve 138 is open. Further,the heating chamber 132 is positioned above the basin 103 so that oncethe heating chamber 132 is filled with liquid by pump 130, the fillingof the basin 103 from the heating chamber 132 may be done solely by theinfluence of gravity. Accordingly, the operation of the densitometer infilling and emptying the basin 103 is simple and extremely quiet.

[0109] In those situations where plumbing is readily accessible, eitheror both of the supply and drain tanks 124 and 126 may be bypassed anddirect connections made to existing drains or supply lines.Specifically, the pump 130 may be replaced with a valve (not shown)connecting the heating chamber 132 to the water supply line. Conversely,the connection between the T-connector 144 and the drain tank 126 mayre-routed to connect the T-connector 144 directly to a drain.

[0110] Even with the constant refreshing of the coupling liquid in thebasin 103 by the liquid handling system of the present invention, theliquid contacting surfaces of the basin 103, the heating chamber 132,the valves 138 and 148, and the connecting tubing are susceptible tobacterial colonization and to encrustation by minerals. The coatings ofcolonization or encrustation are potentially unhygienic andunattractive. Sufficient build-up of minerals or bacteria may alsoadversely affect the operation of the densitometer 100 either byrestricting liquid flow through the tubing, by interfering with theoperation of the valves 138 or 148, or by adversely affecting theacoustical properties of the transducer array 121.

[0111] For this reason, the densitometer 100 is desirably periodicallyflushed with an antibacterial solution and a weak acid, the latter toremove mineral build-up. These measures are not always effective or maybe forgotten, and hence, in the present invention critical watercontacting surfaces are treated with a superficial antibacterialmaterial which is also resistant to mineral encrustation. The preferredtreatment is the SPI-ARGENT™ surface treatment offered by the SpireCorporation of Bedford, Mass. which consists of an ion beam assisteddeposition of silver into the treated surfaces. The resulting thin filmis bactericidal, fungistatic, biocompatible, and mineral resistant. Theproperties of being both bactericidal and fungistatic are generallytermed infection resistant.

[0112] This surface treatment is applied to the water contactingsurfaces of the basin 103, the heating chamber 132 and the criticalmoving components of the valves 138 and 148.

[0113] Referring now to FIG. 14, the general arrangement of theelectrical components of FIG. 4 is unchanged in the ultrasonicdensitometer 100 of FIG. 10 except for the addition of I/O circuitry andcircuitry to control the pump 130, valves 138 and 148, and heatingchamber 132 of the liquid handling system. In particular, microprocessor38 now communicates through bus 40 with an I/O module 174, a pump/valvecontrol circuit 160 and a heater control circuit 162.

[0114] I/O module 174 provides the ability to connect a standard videodisplay terminal or personal computer to the densitometer 100 fordisplay of information to the user or for subsequent post processing ofthe data acquired by the densitometer and thus allows an alternative tomicroprocessor 38 and display 18 for processing and displaying theacquired ultrasound propagation data.

[0115] The pump/valve control circuit 160 provides electrical signals tothe fill valve 138 and the drain valve 148 for opening or closing eachvalve under the control of the microprocessor 38. The pump/valve controlcircuit 160 also provides an electrical signal to the pump 130 to causethe pump to begin pumping water and surfactant from the supply tank 124under the control of microprocessor 38, and receives the signal from thelevel sensor 172 in the heating chamber 132 to aid in the control of thepump 130 and valve 138.

[0116] The heater control circuit 162 controls the current received bythe resistive heating element 164 and also receives the signal from athermistor 168 in thermal contact with the heating chamber 132. A secondthermistor 170, positioned in basin 103 to be thermal contact with theliquid in that basin 103, is also received by the heater control circuit162.

[0117] Referring now to FIGS. 13 and 14, during operation of thedensitometer 100 and prior to the first patient, the basin 103 will beempty, the supply tank 124 will be filled and contain a known volume ofwater and surfactant, and the drain tanks 126 will be empty. Both manualvalves 128 will be open to allow flow into or out of the respectivetanks 124 and 126 and the electrically actuated fill valve 138 and drainvalve 148 will be closed.

[0118] Under control of microprocessor 38, the pump/valve controlcircuit 160 provides current to the pump 130 which pumps water andsurfactant upward into heating chamber 132 until a signal is receivedfrom level sensor 172. When the heating chamber 132 is filled to theproper level as indicated by level sensor 172, the signal from levelsensor 172 to pump/valve control circuit 160 causes the pump 130 to beturned off. At this time, a predetermined volume of liquid is containedin heating chamber 132 which translates to the proper volume needed tofill basin 103 for measurement.

[0119] Under command of microprocessor 38, the heater control circuit162 provides a current through thermal protection module 166 toresistive heating element 164. The temperature of the liquid in theheating chamber 132 is monitored by thermistor 168 and heating continuesuntil the liquid is brought to a temperature of approximately 39° C. Thethermistor and a thermal fuse (not shown) of the thermal protectionmodule 166 provide additional protection against overheating of theliquid. The thermistor opens at 50° C. and resets automatically as itcools and the thermal fuse opens at 66° C. but does not reset and mustbe replaced. The opening of either the thermistor or the thermal fuseinterrupts current to the resistive heating element 164.

[0120] When the liquid in the heating chamber 132 is brought to thecorrect temperature, fill valve 138 is opened by microprocessor 38,through pump/valve control circuit 160, and liquid flows under theinfluence of gravity into the basin 103 at the proper temperature. Thecontrol of the temperature of the liquid serves to insure the comfort ofthe patient whose foot may be in the basin 103 and to decrease anytemperature effects on the sound transmission of the water andsurfactant.

[0121] Once the heated liquid has been transferred from the heatingchamber 132 to the basin 103, the fill valve 138 is closed and the pump130 is reactivated to refill the heating chamber 132. Thus, fresh liquidfor the next measurement may be heated during the present measurement toeliminate any waiting between subsequent measurements.

[0122] With liquid in place within the basin 103, the measurement of theos calcis by the densitometer 100 may begin. In this respect, theoperation of the ultrasonic densitometer of FIG. 10 is similar to thatof the embodiment of FIG. 1 except that the order of pulsing andmeasurement can be varied. In the apparatus of FIG. 1, the measurementpulse through the member was generally performed before the referencepulse through homogenous standard, i.e. water. In the densitometer 100of FIG. 10, since the distance between the transducers 121 is fixed, thereference pulse through the homogenous standard material, which issimply the liquid in basin 103, may be conducted before or after ameasurement pulse through a live member is performed. In fact, becausethe temperature of the liquid in the basin 103 is held steady by thetemperature control mechanism as described, the standard transmit timemeasurement can be made once for the instrument and thereafter onlymeasurement pulses need be transmitted.

[0123] Preferably, the standard transit time measurement is stored as anumber in the memory of microprocessor 38 during the initial calibrationof the unit at the place of manufacture or during subsequentrecalibrations. During the calibration of the densitometer 100, thesignal from the thermistor 170 is used to produce a transit timecorrected for the temperature of the liquid according to well knownfunctional relations linking the speed of sound in water to watertemperature. It is this corrected transit time that is stored in thememory associated with microprocessor 38 as a stored standard reference.

[0124] The transit time of the measurement pulses is compared to thestored standard reference transit times through the coupling liquid togive an indication of the integrity of the member just measured. Thus,one may dispense with the reference pulse entirely. Empirical tests havedetermined that by proper selection of a standard reference value storedin the memory of microprocessor 38 and by holding the liquid in thebasin within a temperature range as provided by the heating chamber 132,no reference pulse need be launched or measured.

[0125] Using this variation, a mathematical comparison of the measuredtransit time, or transit velocity, must be made to the standard. Since,in the interests of accuracy, it is preferred to use both changes intransit time (velocity) and changes in attenuation to evaluate a memberin vivo, the following formula has been developed to provide a numericalvalue indicative of the integrity and mineral density of a bone:

bone integrity value=A(SOS-B)+C(BUA-D)  (1)

[0126] In this formula, “SOS” indicates the speed of sound, or velocity,of the measurement ultrasonic pulse through the member, and is expressedin meters per second. The speed of sound (SOS) value is calculated fromthe measured transit time by dividing a standard value for the memberwidth by the actual transit time measured. For an adult human heel, ithas been found that assuming a standard human heel width of 40 mm at thepoint of measurement results in such sufficient and reproducibleaccuracy that actual measurement of the actual individual heel is notneeded.

[0127] BUA is broadband ultrasonic attenuation, as described in greaterdetail above. The constants A, B, C, and D offset and scale theinfluence of the BUA measurement relative to the SOS measurement toprovide a more effective predictor of bone density. These constants maybe determined empirically and may be selected for the particular machineto provide numbers compatible with dual photon absorptiometry devices,such as an estimated bone mineral density (BMD) value, and to reducebone width effects. Since this method utilizing ultrasonic measurementof the heel is quick and free from radiation, it offers a promisingalternative for evaluation of bone integrity. It will be understood thatmultiple SOS and BUA values may be obtained, averaged, then combined perthe above formula, or that each SOS and BUA value may be combined toproduce a bone integrity value and the multiple bone integrity valuesthen averaged.

[0128] The densitometer 100 may be used with or without an array ofultrasonic transducers in the transducers 121. In its simplest form themechanical alignment of the heel in the device can be provided by theshape and size of the basin 103. While the use of an array, andregion-of-interest scanning as described above, is most helpful inensuring a reproducible and accurate measurement, mechanical placementmay be acceptable for clinical utility, in which case only singletransducer elements are required.

[0129] Upon completion of the measurement, the drain valve 148 is openedby microprocessor 38 through pump/valve control circuitry 160, and theliquid in the basin 103 is drained through “T” 144 to the drain tank126. At the beginning of the next measurement, the drain valve 148 isclosed and liquid is again transferred from the heating chamber 132 ashas been described.

[0130] With repeated fillings and drainings of the basin 103, the levelof liquid in the fill tank 124 decreases with a corresponding increasein the level of the liquid in the drain tank 126. The height of theliquid in each tank 124 and 126 may be tracked by a conventional levelsensor such as a mechanical float or a capacitive type level sensor.

[0131] Preferably no additional level sensor is employed. The volume ofliquid for each use of the densitometer 100 is known and defined by thefill level of the heating chamber 132. The microprocessor 38 maytherefore track the level of liquid remaining in the supply tank 124 bycounting the number of times the basin 103 is filled to provide a signalto the user, via the display 18 or a remote video display terminal (notshown), indicating that the tanks 124 and 125 need to be refilled anddrained respectively. This signal to the user is based on the number oftimes the basin 103 is filled and a calculation of the relative volumesof the heating chamber 132 and supply tank 124.

[0132] After completion of the use of the densitometer 100 for a periodof time, the densitometer may be stored. In a storage mode, after boththe supply tank 124 and drain tank 126 have been manually emptied, themicroprocessor 38 instructs the pump/valve control circuit 160 to openboth the fill valve 138 and the drain valve 148 and to run the pump 130.The drain valve 138 is opened slightly before the pump 130 is actuatedto prevent the rush of air from causing liquid to flow out of theoverflow port 134.

[0133] Referring now to FIGS. 10 and 15, the transducers 121 areinserted into the basin 103 through tubular sleeves 180 extendingoutward from the walls of the basin 103 at the curved pocket along anaxes 212 of the opposed transducers 121. The tubular sleeves 180 definea circular bore in which the transducers 121 may be positioned. Eachtransducer 121 seals the sleeve 180 by compression of o-ring 182positioned on the inner surface of the sleeve 180.

[0134] Although the transducers 121 fit tightly within the sleeves 180,their separation and alignment are determined not by the sleeves 180 butby an independent C-brace 184 comprising a first and second opposed arm186 separated by a shank 188. A transducers 121 is attached to one endof each of the arms 186, the other ends of the arms 186 fitting againstthe shank 188.

[0135] The arms 186 are generally rectangular blocks transversely boredto receive the cylindrically shaped transducers 121 at one end and tohold them along axis 212. The other ends of the arms 186 provide planarfaces for abutting the opposite ends of the block like shank 188, theabutting serving to hold the arms 186 opposed and parallel to eachother.

[0136] Although the angles of the arms 186 with respect to the shank 188are determined by the abutment of the planar faces of the arms 186 andthe ends of the shank 188, alignment of the arms 186 with respect to theshank 188 is provided by dowel tubes 190 extending outward from each endof the shank 188 to fit tightly within corresponding bores in the firstand second arm 186.

[0137] Cap screws 194 received in counterbored holes in the arms 186pass through the arms 186, the dowel tubes 190 are received by threadedholes in the shank 188 to hold the arm 186 firmly attached to the shank188. The dowel tubes 190 and surfaces between the arms 186 and shank 188serve to provide extremely precise alignment and angulation of thetransducers 121, and yet a joint that may be separated to permit removalof the transducers 121 from the densitometer 10 for replacement orrepair.

[0138] Transducers 121 are matched and fitted to the arms 186 in acontrolled factory environment to provide the necessary acoustic signalstrength and reception. In the field, the shank 188 may be separatedfrom one or both arms 186 by loosening of the cap screws 194 so as toallow the transducers 121 extending inward from the arms 186 to be fitwithin the sleeves 180. Proper alignment and angulation of thetransducers is then assured by reattaching the arm or arms 186 removedfrom the shank 188 to the shank 188 to be tightened thereto by the capscrews 194. Thus, the alignment of the transducers is not dependent onthe alignment of the sleeves 180 which may be molded of plastic and thusbe of relatively low precision. Nor must alignment be tested while thetransducers are in the sleeves 180 attached to the basin 103 but may bechecked in a central controlled environment.

Flexible Bladder with Moving Supports

[0139] Referring now to FIGS. 16 and 17, in yet another embodiment ofthe present invention, the opposed transducers 121 are fitted withannular collars 200 which in turn are attached to flexible bladders 202extending inward to the basin 103, each bladder 202 containing a liquidor semi-liquid coupling material 204.

[0140] The bladders 202 serve to contain the gel about the face of thetransducers 121 and conform to the left and right sides of a patient'sheel 207, respectively, to provide a path between the transducers 121and the soft tissue and bone of the heel 207 without intervening air.The bladder 202 further prevents the coupling material from directcontact with the heel to permit selection of the coupling material 204from a broader range of materials.

[0141] Compression of the bladders 202 against the heel 207, so as toprovide the necessary coupling, is provided by a telescoping shank 181shown in FIG. 16. In this alternative embodiment of the C-brace 184 ofFIG. 15, the shank 188′ has been cut into two portions 206 and 208slidably connected together by dowel pins 210 to provide necessarymotion of the transducers 121 inward along their axis to compress thebladders 202 against the heel 207. One end of each dowel pin 210 ispress fit within bores in the shank 188′ parallel to the axis 212 of theopposed transducers in portion 206. The other ends of the dowel pins 210slide within larger bores in portion 208 so that portions 208 and 206may slide toward and away from each other parallel to the axis 212. Withsuch motion, the attached arms 186 move towards and away from each otheradjusting the separation of the transducers 121 between an open positionfor insertion of the heel 207 and a closed position of known separationand orientation where portions 208 and 206 abut.

[0142] Control of the separation is provided by means of cam pins 214protruding from portions 206 and 208 on the side away from the extensionof the arms 186 and generally perpendicular to the axis 212. These pins214 are received by spiral shaped slots in a cam disk 217 fitting overthe cam pins 214. The disk includes radially extending lever 218 whosemotion rotates the disk causing the cam pins 214 within the slots 215 tobe moved together or apart depending on motion of lever 218.

[0143] Thus, the transducers 121 may be moved apart together with thebladders 202 for insertion of the heel 207 into the basin 103. Once theheel is in place, motion of the lever 218 closes the transducers 121 toa predetermined fixed separation compressing the bladders 202 snuglyagainst the sides of the heel 207. The elasticity of the bladder filledwith coupling material 204 provides an expanding force against the heel207 to closely conform the surface of the bladder 202 to the heel 207.

[0144] The collars 200 may provide a conduit for electrical wiring (notshown) including wiring attached to a temperature sensor for monitoringthe temperature of the contained coupling material 204 and heaterelements for heating the temperature of the contained coupling materialto a predetermined temperature. As has been described above, thecontrolled heating provides both for comfort to the patient and forreproducibility of the measurements which may be influenced by thetemperature of the contained coupling material 204.

Cancellation of Heel Width Variations

[0145] Referring to FIGS. 17 and 18, generally the thicker the calcaneus216 of the heel 207, the greater the attenuation of an acoustic signalpassing through the heel 207 between transducers 121. Correspondingly,with greater attenuation, the slope of attenuation as a function offrequency, generally termed broadband ultrasonic attenuation (BUA) alsoincreases as shown generally in FIG. 18 by plot 209. This assumesgenerally that the coupling medium 204 is of low or essentially constantattenuation as a function of frequency. Greater BUA is generallycorrelated to higher bone quality.

[0146] For constant heel thickness, lower TOF (faster sound speed)corresponds generally to higher bone quality. The time of flight (TOF)of an acoustic pulse between the transducers 121 will be proportional tothe time of flight of the acoustic pulse through regions A of FIG. 17comprising the path length through coupling gel 204, regions Bcomprising the path length through soft tissue of the heel 207surrounding the calcaneus 216, and region C comprising the path lengththrough the heel bone or calcaneus 216. Thus, $\begin{matrix}{{TOF} = {{\frac{1}{V_{A}}A} + {\frac{1}{V_{B}}B} + {\frac{1}{V_{C}}C}}} & (2)\end{matrix}$

[0147] where V_(A), V_(B), and V_(C) are the average speed of soundthrough the coupling gel, soft tissue and bone respectively and A, B, Care the path lengths through these same materials. Provided that theseparation between the transducers 121 is a constant value K, then timeof flight will equal: $\begin{matrix}{{TOF} = {{\frac{1}{V_{A}}\left( {K - C - B} \right)} + {\frac{1}{V_{B}}B} + {\frac{1}{V_{C}}C}}} & (3)\end{matrix}$

[0148] The change in time of flight as a function the thickness of thebone C (the 5 derivative of TOF with respect to C) will thus generallybe equal to: $\frac{1}{V_{C}} - {\frac{1}{V_{A}}.}$

[0149] Referring now to FIG. 18, if the velocity of sound through thecoupling medium 204 is greater than that through the bone being measured$\left( {{V_{A} > V_{C}},\quad {{{or}\quad \frac{1}{V_{C}}} > \frac{1}{V_{A}}}} \right),$

[0150] then the functional relationship of TOF to heel width will be oneof increasing as the heel becomes wider (indicated at plot 213 showingvalues of 1/TOF). On the other hand, if the velocity of sound throughthe coupling medium 204 is less than that through the bone beingmeasured$\left( {{V_{C} > V_{A}},\quad {{{but}\quad \frac{1}{V_{A}}} > \frac{1}{V_{C}}}} \right),$

[0151] then the functional relationship of TOF to heel width will be oneof decreasing as the heel becomes wider (indicated at plot 211 showingvalues of 1/TOF).

[0152] A combined bone quality figure may be obtained by combining BUAand 1/TOF measurements (1/TOF because BUA increases but TOF decreaseswith denser bone). Further, if (1) the conditions of ultrasonicpropagation are adjusted so that the slope of 1/TOF with heel width isopposite in sign to the slope of BUA with heel width (i.e., V_(A)>V_(C)) and (2) the BUA and 1/TOF measurements are weighted with respectto each other so that the opposite slopes of the BUA and 1/TOF areequal, then the algebraic combination of the BUA and TOF, throughaddition for example, will produce a bone quality measurementsubstantially independent of heel width for a range of bone qualities.

[0153] This can be intuitively understood by noting that as the heelgets wider, it displaces some of the coupling gel 204 from between theheel 207 and each transducer 121, and by displacing material thatconducts sound slower than the bone being measured increasing the totalspeed with which the sound is conducted.

[0154] Note that a similar effect may be obtained by proper scaling andcombination of BUA and TOF by multiplication and that other functions ofattenuation and TOF could be used taking advantage of their functionalindependence and their functional dependence in part on heel width.

[0155] Referring now to FIG. 19, generally BUA and TOF are functionallyrelated to both bone quality and bone width. It should be possible,therefore, to solve the equations governing these relationships for bonequality alone and thus to eliminate the effect of the common variable ofheel width. With such an approach, the variable of heel width iseliminated not just for a portion but through the entire range of bonemeasurement provided that the coupling medium is different from the bonebeing measured so that there will be a width effect in both BUA and TOFmeasurements.

[0156] Approximations of the algebraic relationships describing thefunctional dependence of BUA and TOF on bone quality and bone width, canbe obtained through the construction of a set of bone phantoms ofdifferent widths and bone qualities when using a particular couplinggel. Generally, for each value of BUA or TOF the data will describe acurve 222 linking that value with different combinations of bone qualityand bone width. This data may be placed in a look-up table in the memoryof the microprocessor of the densitometer as has been previouslydescribed.

[0157] After BUA and TOF values are determined, the data of the look-uptable (comprising many bone quality and bone width pairs for each of thedetermined BUA and TOF values) are scanned to find a bone quality andwidth data pair for the BUA value matching a bone quality and width datapair for the TOF value. This is equivalent to finding the intersectionof the two curves 222 associated with the measured BUA and TOF values.The matching bone quality values of the data base will give a bonequality having little or no bone width influence. This value may bedisplayed to the clinician. It is noted that the previously describedtechnique of summing weighted values of BUA and 1/TOF is but aspecialized form of this process of algebraic solution.

[0158] Alternatively, a matching bone width value can be identified,being the width of the measured heel, and used to correct either of theBUA or TOF values for display to the clinician in circumstances whereBUA or TOF values are preferred for diagnosis.

[0159] This ability to cancel out heel width effects will work only forbone qualities where the relationship between the coupling gel 204 andthe calcaneus 216 are such as to provide a functional dependence on heelwidth. Cancellation will not occur, for example, if the density of thecalcaneus 216 being measured is substantially equal to the sound speedof the coupling gel 204 and thus where displacement of the coupling gelby similar bone will have no net effect on time of flight. Thus thecoupling gel must be properly selected. In this case, materials havinghigher sound speed may be selected for the coupling material. Thedifference between the coupling gel and the bone being measured willinfluence the accuracy of the cancellation of heel width effects.

[0160] Moderating this desire to improve heel width effects is theimportance of keeping the coupling gel 204 close to the acousticproperties of the soft tissue of the heel 207 both to prevent reflectionby impedance mismatch and to prevent variations in the thickness of thesoft tissue in regions B from adding additional uncertainty to themeasurement. The coupling medium of water provides good matching to thesoft tissue of the heel 207 and has a sound velocity very close to boneand some osteoporotic conditions. Weighting of the attenuation andpropagation time may be made for water.

[0161] Although the preferred embodiment of the invention contemplatesdisplay of a bone quality value or corrected TOF or BUA values, it willbe recognized that the same effect might be had by displayinguncorrected BUA or TOF values on a chart and establishing a thresholdfor strong or weak bone based on the corrections determined as above.

Ultrasonic Densitometer with Scannable Focus

[0162] Referring now to FIG. 20, a receiving transducer array 300,similar to array 21 described with respect to FIG. 1, may be positionedadjacent to the heel of a patient (not shown) to receive an ultrasonicwave 410 along axis 304. The receiving transducer array 300 includes apiezoelectric sheet 302 of substantially square outline positionednormal to the transmission axis 304 and is divided into transducerelements 400 as will be described, each which receives a differentportion of the ultrasonic wave 410 after passage through the heel.

[0163] The piezoelectric sheet 302 may be constructed of polyvinylidenefluoride and has a front face 306 covered with a grid of interconnectedsquare electrodes 308 deposited on the front face by vacuummetallization. These square electrodes 308 are arranged at theinterstices of a rectangular grid to fall in rectilinear rows andcolumns. Referring also to FIG. 23, each square electrode 308 is spacedfrom its neighboring electrodes 308 by approximately its width. Thesesquare electrodes 308 are Connected together by metallized traces (notshown) and to a common voltage reference by means of a lead 310.

[0164] In the manufacture of the piezoelectric sheet 302, thepolyvinylidene sheet is polarized to create its piezoelectric propertiesby heating and cooling the sheet in the presence of a polarizingelectrical field according to methods generally understood in the art.In the present invention, this polarizing field is applied only to thearea under the square electrodes 308 so that only this material ispiezoelectric and the material between square electrodes 308 has reducedor no piezoelectric properties. As will be understood below, thisselective polarization of the piezoelectric sheet 302 provides improvedspatial selectivity in distinguishing between acoustic signals receivedat different areas on the piezoelectric sheet.

[0165] Referring now to FIG. 22, opposite each electrode 308 on the backside of the piezoelectric sheet 302 furthest from the source of theultrasonic wave 410 is a second electrode 312 having substantially thesame dimensions as the square electrodes 308 and aligned withcorresponding square electrodes 308 along transmission axis 304.

[0166] Referring to FIGS. 20 and 22a, a connector board 318 of arealdimension substantially equal to the piezoelectric sheet 302 has,extending from its front surface, a number of conductive pins 320corresponding to the pads 316 in number and location. The pins 320 arestake-type terminals mounted to an epoxy glass printed circuit board 322of a type well known to those of ordinary skill in the art. Eachconductive pin 320 is connected directly to a preamplifer and then bymeans of printed circuit traces to a multiplexer 325 to a reduced numberof control and data lines 324 which may be connected to themicroprocessor 38 of the densitometer through an A to D converter 42described previously with respect to FIG. 1 and as is well understood inthe art. The preamplifers allow grounding of those electrodes 312 notactive during scanning to reduce cross-talk between electrodes 312.

[0167] As shown in FIG. 22a, the pins 320 of the connector board 318 areelectrically connected to electrodes 312 on the back surface of thepiezoelectric sheet 302 by means of a strip of thin (0.0005″) Mylar 316having conductive fingers 314 on its surfaces. The conductive fingers314 on the front and rear surfaces of the Mylar strip 316 are inelectrical communication through a plated-through hole 313 in the Mylar316 connecting the fingers 314.

[0168] Each conductive pin 320 is attached to a conductive finger 314 atone edge of the Mylar strip 316 at the rear of the Mylar strip 316(according to the direction of the acoustic wave) by means of ananisotropically conductive adhesive film 315 providing electricalconduction only along its thinnest dimension, thus from pin 320 tofinger 314 but not between fingers 314 or pins 320. Anisotropicallyconductive film suitable for this purpose is commercially available from3M corporation of Minnesota under the trade name of 3M Z-Axis AdhesiveFilm.

[0169] The other end of each plated finger 314 on the front of the Mylarstrip 316 is then connected to an electrode 312 by a second layer ofanisotropically conductive adhesive film 317. The Mylar strip 316 flexesto allows the pins 320 to be spaced away from the electrode 312 toreduce reflections off the pins 320 such as may cause spurious signalsat the piezoelectric sheet 302. The Mylar strip 316 and conductivefingers 314 are essentially transparent to the acoustic wave.

[0170] Referring to FIG. 22b, the Mylar strips 316 and adhesive film 315and 317 allow rapid assembly of the transducer 300. A single layer ofconductive film 317 (not shown in FIG. 22b) may be applied over theentire rear surface of the piezoelectric sheet 302 and electrodes 312.Next a plurality of overlapping Mylar strips 316 may be laid down uponthis surface, each Mylar strip 316 extending laterally across thepiezoelectric sheet 302 with transversely extending conductive fingers314 for each electrode 312 of one row of conductive electrode 312. Theoverlapping of the Mylar strips 316 ensures that only a front edge ofeach strip 316 adheres to the piezoelectric sheet 302. Guide holes 319in the laterally extreme edges of the Mylar strips 316 fit into pins ina jig (not shown) to ensure alignment of fingers 314 with electrodes312.

[0171] Next, a second layer of the anisotropically conductive adhesivefilm 315 is placed on the rear surfaces of the overlapping Mylar strips316 and the conductive pins 320 pressed down on this film 315, alignedwith the other ends of the conductive fingers 314 to attach to theirrespective fingers 314. The conductive pins 320 are then raised andfixed in spaced apart relationship with the piezoelectric sheet 302, theMylar strips 316 flexing to accommodate this displacement.

[0172] The ultrasonic wave 410 passing through portions of thepiezoelectric sheet 302 between electrodes 308 and 312 may thereby bemeasured at a number of points over the surface of the piezoelectricsheet by the electric signals generated and collected by electrodes 308and 312 according to multiplexing methods well known in the art. Eachelectrode pair 308 and 312 provides an independent signal of theacoustic energy passing through the area of the piezoelectric sheet 302embraced by the electrode pair.

[0173] A protective frame 325 encloses the piezoelectric sheet 302 andconnector board 318 protecting them from direct contact with water ofthe basin 103 shown in FIGS. 10 and 15 into which the receivingtransducer array 300 may be placed. The frame 325 holds on its frontface an acoustically transparent and flexible material 326 such as aTeflon film so that the ultrasonic wave 410 may pass into the frame toreach the piezoelectric sheet 302.

[0174] The above described array may be used either to receive ortransmit acoustic waves and is not limited to use in the medical areabut may provide an inexpensive and rugged industrial acoustic arrayuseful for a variety of purposes including industrial ultrasonic imagingand the construction of high frequency synthetic aperture microphones.

[0175] Positioned behind the frame 325 is an electric motor 328 drivinga central gear 330 about an axis aligned with transmission axis 304 andapproximately centered within the frame 325. The central gear 330 inturn engages two diagonally opposed planet gears 332 also turning aboutaxes aligned with the transmission axis. Each planet gear 332 has a rod334 extending forwardly from a front face of the planet gear 332 butoffset from the planet gear's axis to move in an orbit 336 thereabout.The orbit 336 has a diameter approximately equal to the spacing betweenelectrodes 308.

[0176] The rods 334 engage corresponding sockets 338 on the back side ofthe frame 325 at its opposed comers. Thus activation of the motor 328causes the piezoelectric sheet 302 and connector board 318 to follow theorbit 336 while maintaining the rows and columns of detector elements400 in horizontal and vertical alignment, respectively.

[0177] Referring now to FIG. 23, a sampling of the signals from thedetector elements 400 may be made at four points 342 in the orbit 336 atwhich each electrode 308 is first at a starting position, and then ismoved half the inter-electrode spacing upward, leftward, or upward andleftward. The effect of this motion of the detector elements 400 is todouble the spatial resolution of the received acoustic signals withoutincreasing the amount of wiring or the number of detector elements 400.The sampling of acoustic energy at each of the points 342 is stored inthe memory of the microprocessor and can be independently processed toderive attenuation, BUA or time of flight measurements or a combinationof these measurements. These measurements are then converted to anintensity value of an image so that each pixel of the image has anintensity value proportional to the measured parameter. A clinicianviewing the image thus obtains not merely an image of the bone, but animage that indicates bone quality at its various points.

[0178] A transmitting ultrasonic transducer 408 is positioned oppositethe receiving transducer array 300 from the heel 207 and produces agenerally planar ultrasonic wave 410 passing into the heel. Generally,the acoustic signal received by each transducer element 400 will havearrived from many points of the heel.

[0179] Referring now to FIG. 24, if the transducer elements 400 werefocused as indicated by depicted transducer elements 400′ to follow ahemisphere 402 having a radius and hence focus at a particular volumeelement or voxel 404 within the heel, acoustic signals from other voxelscould be canceled providing greater selectivity in the measurement. Inthis focusing of the transducer elements 400′, the signals from each ofthe elements 400′ are summed together. Constructive and destructiveinterference of ultrasonic waves 410 from the heel 207 serve toeliminate acoustic signals not flowing directly from focus volumeelement 404.

[0180] For example as depicted, two acoustic signals 405 and 406 fromfocus voxel 404 both crest at the location of a transducer element 400′as a result of the equidistance of each transducer element 400′ fromfocus voxel 404. When the signals from transducer elements 400′ aresummed, the signal from focus voxel 404 will increase. In contrast,acoustic signals from other voxels not equidistant to transducerelements 400′ will tend to cancel each other when summed and thusdecrease.

[0181] The present invention does not curve the transducer elements 400into a hemisphere but accomplishes the same effect while retaining thetransducer element 400 in a planar array by delaying the signalsreceived by the transducer elements 400 as one moves toward thecentermost transducer element 400″ so as to produce an effectivehemispherical array. Like a hemispherical array, the center-mosttransducer elements 400 appear to receive the acoustic wave a littlelater than the transducer elements 400 at the edge of the receivingtransducer array 300. By using a phase delay of the signals instead ofcurving the receiving array 300, the position of the focus voxel 404 atwhich the receiving array 300 is focused, may be scanned electrically aswill be described. The signals from each of the transducer elements 400are received by the A/D converter 42 and stored in memory. Phaseshifting as described simply involves shifting the point at which onestarts reading the stored signals.

[0182] Adjusting the phase of the acoustic signals received by each ofthe transducer elements 400 allows the location of the focus voxel 404from which data is obtained to be scanned through the heel. The phase issimply adjusted so that the effective arrival time of an acoustic signaloriginating at the desired location is the same for each of thetransducer elements 400.

[0183] Referring now to FIG. 25, the location of focus voxel 404 may bemoved in a first and second raster scan pattern 412 and 414 (as readingsare taken over many ultrasonic pulses) to obtain separated planes ofdata normal to the transmission axis 304. The first plane of data 412may, for example, be positioned near the outer edge of the os calcis 216to measure the cortical bone quality while the second plane 414 may beplaced in a centered position in the trabecular bone to obtain asomewhat different reading, both readings providing distinct data aboutthe bone.

[0184] It will be understood that this same approach of scanning indifferent planes may be used to obtain a volume of data within the heel207, in this case, the focus voxel 404 being moved to points on a threedimensional grid.

[0185] In another embodiment (not shown) the transmitting ultrasonictransducer may be an array and the phases of the ultrasonic signalstransmitted by each of the elements of the array may be phased so as tofocus on a particular voxel within the heel. In this case, the receivingarray may be a single broad area detector or may also be an arrayfocused on the same voxel for increased selectivity. The focus point ofthe transmitting and receiving arrays may also be shifted with respectto each other to investigate local sound transfer phenomenon. As before,the focal points of either array may be steered electrically by themicroprocessor through a shifting of the phases of the transmitted andreceived signals. To collect data, each element of the transmit arraymay be energized individually while all receive elements of the receivearray are read. This may be continued until each of the elements of thetransmit array have been energized.

[0186] Alternatively, referring to FIG. 28, the receiving array 300 maybe actually formed so that its elements follow along the hemisphere 402so as to have a fixed focus on focus voxel 404. Additional circuitry toeffect the phase adjustment needed to focus the array is not needed inthis case. The receiving array 300 is attached to an X-Y-Z table 600providing motion in each of three Cartesian axes under the control ofthe microprocessor via stepper motors 610. At each different location ofthe table 600, data may be collected from focus voxel 404 to establishthe data points on the three dimensional grid. The transmitting array408 may be held stationary or may be moved with the scanning of thereceiving array 300 and may be focused as well.

[0187] Referring now to FIG. 26, such a data volume 415 may include aplurality of data voxels 416 each providing a measured member parameterfor the bone or tissue at that point in the heel. A point of minimumbone density 418 may be found within this data volume 415 and used toidentify a region of interest 420 which will serve as a standard regionfor measuring the bone density of the heel. This region may beautomatically found after collection of the data volume 415 and onlythose voxels 416 within the region of interest 420 may be used for adisplayed measurement. This automatic location of a region of interest420 provides a much more precise bone characterization.

[0188] Acquiring a data volume 415 also provides the opportunity to usethe extra data outside the region of interest 420 to ensure that thesame region of interest 420 is measured in the patient's heel over aseries of measurements made at different times. The data volume 415 maybe stored in memory as a template that may be matched to subsequentlyacquired data volumes. The region of interest 420 spatially located withrespect to the first template, may then be used as the region ofinterest for the subsequent data volumes aligned with that template toprovide more repeatability in the measurement.

[0189] Referring now to FIG. 27 in such a template system in a firststep 500, a collection of a data volume 415 within the heel is obtained.At decision block 502, if this is a first measurement of a particularpatient, a region of interest 420 is identified at process block 504from this data, as a predetermined volume centered about a point ofminimum bone density 418 as described with respect to FIG. 26. Atprocess block 506, the data volume is stored as a template along withthe region of interest defined with respect to the data of the template.

[0190] Referring again to decision block 502 on a subsequent measurementof a patient, the program may proceed to process block 508 and thetemplate previously established may be correlated to a new data volume415 collected at process block 500. The correlation process involvesshifting the relative locations of the two data volumes to minimize adifference between the values of each data voxel 416 of the datavolumes. In most situations, this will accurately align the two datavolumes so that corresponding voxels 416 of the two data volumes 415measure identical points within the patient's heel. The region ofinterest 420 associated with the template is then transferred to the newdata volume as it has been shifted into alignment with the template sothat the identical region of interest may be measured in a patient evenif the patient's foot has taken a different alignment with respect tothe transducer array 300 and 408. This use of the template's region ofinterest 420 is indicated by process block 510.

[0191] At process block 512, an index is calculated at the region ofinterest 420 for the new data volume 415 being typically an averagevalue of a bone parameter such as BUA or time of flight for the voxels416 within the region of interest 420. This data is then displayed tothe clinician at process block 520 as has been described.

Overlapping Bladder Embodiment

[0192] Referring now to FIG. 29, in an alternative embodiment, atransducer array 300, as has been described above, or an array 21,described with respect to FIG. 3, may extend into the basin 103 which isno longer filled with water. Instead, the transducer array 300 iscoupled to the heel 207 through three concentric overlapping bladderwalls.

[0193] The first, internal bladder 522 is supported about the transducerarray 300 by means of an annular collar 524 to provide an enclosedvolume in much the same manner as has been described with respect to thebladder 202 of FIG. 17 with the exception that the annular collar 524includes an orifice 526 allowing a first coupling liquid to beintroduced into the bladder 522 through delivery pipe 528 as driven bypump 130B. A branch connection 530 to delivery pipe 528 provides thesame coupling liquid to an identical and opposing transducer/bladderassembly on the other side of the heel, not shown in FIG. 29.

[0194] A heat exchanger 532, as is well understood in the art, couplesto the delivery pipe 528 to ensure that the coupling liquid 534 is at aconstant predetermined temperature to which the device has beenpreviously calibrated. The heat exchanger 532 may make use of acirculating exchanger liquid preheated to a constant temperature bymeans of a combination thermostat and electric heater (not shown) wellknown in the art. A pressure transducer 536 communicating with thedelivery pipe 528 measures the pressure of the coupling liquid 534 andis used to control the pump 130B so that the bladder 522 is inflatedonly to a predetermined pressure. This ensures a repeatable andpredetermined deformation of the soft tissue of the heel 207 andprovides, to the extent possible, a reproducible and constant couplingbetween the transducer array 300 and the, heel 207.

[0195] Coaxially about annular collar 524 is a second annular collar 538supporting on its outer edge an second bladder 540 providing a secondenclosed volume between itself and bladder 524 to hold a second couplingliquid 542. Second coupling liquid 542 differs from coupling liquid 534by having either significantly different ultrasonic attenuationcharacteristics or a significantly different sound speed characteristicsfor reasons as will be describe below. The annular collar 538 holdingthe second bladder 540 is also pierced by an orifice 544 connecting to adelivery pipe 546, which like delivery pipe 528, passes through a heatexchanger 550 supplied with the same liquid at the same temperature asheat exchanger 532. Delivery pipe 546 also communicates with pressuretransducer 536 through a coupler 552 which prevents intermingling of theliquids in delivery pipe 528 and 546. Delivery pipe 546 includes abranch connection 554, which like branch connection 530, provides acomparable liquid at a comparable temperature to the opposed transducernot shown in FIG. 29. A separate pump 130A delivers the coupling liquid542 from a distinct reservoir from that connected to pump 130B.

[0196] The bladders 522 and 540 for both transducers are sufficientlyflexible and have ample size so that either bladder 522 or 540 throughinflation to a pressure less than the predetermined pressure controlledby pressure transducer 536 will bridge any gap between the transducerarray 300 and the heel 207, thus allowing the transducer arrays 300 tobe fixed in separation. This both simplifies mechanical construction andeliminates errors resulting from uncertainty about the separationdistance of the transducer arrays 300 as is inherent in any movablesystem.

Bladder Replacement Mechanism

[0197] Referring now to FIGS. 29 and 32, outside of annular collar 538is yet another annular locking collar 556 mounted along with the otherannular collars 538 and 524 against one wall of the basin 103 about thetransducer arrays 300. Locking collar 556 includes radially outwardextending tabs 558 which engage notches 560 in a stretcher ring 562which may be installed against and removed from locking collar 556 bypassing the notches 560 over the tabs 558 and giving the stretcher ring562 a partial clockwise turn to move the locking tabs 558 into undercutportions 564, thereby retaining stretcher ring 562 against the wall ofthe wall of basin 103 against outward forces away from the transducerarray 300. The stretcher ring is covered with a third flexible bladder566 which envelopes both bladders 522 and 540 and yet which is easilyremoved and may be disposed of to provide for hygienic reuse of bladder540 and 522. Because the third flexible bladder 566 is not required toretain a liquid and is disposable, it may be made from a lighter andless resilient material.

Second Width Correction and Extrapolation of Measurement

[0198] The apparatus of FIG. 29 is controlled by means of themicroprocessor 38 (previously described and shown in FIG. 14) havingconnections to the pressure transducer 536 and pumps 130A and 130B asindicated in FIG. 33. Referring now to FIG. 33, at a beginning of ameasurement session, as indicated by process block 568, pump 130B isactivated to inflate inner bladders 522 of opposed transducer arrays300, as shown in FIG. 30, with liquid A to a constant pressure, thusextending the bladders 522 outward against the sides of the heel tooppose the os calcis 216. Any liquid in bladder 540 is withdrawn by pump130A and bladder 566 is vented to allow for the free expansion ofbladder 522.

[0199] At process block 570, series of pulse measurements, as has beendescribed before with respect to the array structures of this invention,are made but with an alternating of the transmitting transducer betweenthe left and right side of the heel 207 with the opposed transducerserving as the receiving transducer. Pairs of such measurements may beaveraged together to reduce the variation in heel measurement, however,it will also be understood that other statistical techniques may beapplied to the left-going and right-going pulses to detect, for example,abnormal situations indicated by the deviation between these pulsesbeing too great.

[0200] At decision block 572 one of three operations may be performed.First, the deviation between successive measurements at process block570 may be compared until the deviations drop below a predeterminedamount indicating that an asymptote in the measurements has beenreached. Alternatively and preferably, the successive ultrasonicmeasurements may be fit to a decaying exponential or other similar curveuntil the deviation between that projected curve and the actual curvedrops below a predetermined amount. Then the asymptote for the projectedcurve may be used as a final value. Alternatively, attenuation and speedof sound measurements may be made at process block 570 and combined toreduce the asymptotic variation in the successive measurements. Thesecombined measurements may also be fit to a curve as previouslydescribed.

[0201] Referring momentarily to FIG. 34, the present inventors haverecognized that in apparatus of this type, the measured values of soundspeed 574 tend to decline with time whereas measured values of broadband ultrasonic attenuation (BUA) 576 tend to rise with time. Acombination of these values reflected in a stiffness measurement 578such as may be an empirically weighted sum of BUA and SOS converges morequickly to the asymptote value 580.

[0202] Referring again to FIG. 33, once an asymptote value for theultrasonic measurements has been determined from which bone healthmeasurements can be made the process may conclude. Alternatively, thecoupling liquid A from bladder 522 is withdrawn and bladder 540 isinflated with coupling liquid B having a different sound speed as shownin FIG. 31 by process block 582. The pressure and temperature of thecoupling liquid B are controlled to be the same as coupling liquid A. Asbefore, with process block 570, interleaved left to right, right to leftultrasonic measurements are then taken as indicated by process block584..

[0203] Next, at decision block 586, an asymptote value is again derived,as has been described, and the program proceeds to process block 588where width of the heel is calculated by the following means:

[0204] Assuming that liquid A has a sound speed of A, liquid B has asound speed of B, the soft tissue about the heel has a sound speed of C,and the os calcis 216 has a sound speed of D, then the time of flightbetween the transducer arrays 300 will be described by the followingequation:

t ₁ =w ₁ /A+w ₂ /C+w ₃ /D  (4)

[0205] where, w₁ is the total distance between the transducer array 300and the heel, w₂ is the total distance through soft tissue, and w₃ isthe total distance through the os calcis 216. Similarly for couplingliquid B, the time of flight will be

t ₂ =w ₁ /B+w ₂ /C+w ₃ /D  (5)

[0206] Accordingly, a difference in time of flight with coupling liquidA and B yields a value of the heel width w₁ as follows::

w₁=(t₁-t₂)/(A-B)  (6)

[0207] If the os calcis is assumed to have a substantially constantwidth, the amount of soft tissue may be deduced and this width of softtissue or the heel width itself may be used to make an empiricalcorrection to the bone health measurement as previously described.

[0208] It will be understood from this description that the sameprocedure may be adopted using coupling liquids A and B that havedifferent attenuations and in fact liquids having both variations inattenuation and sound speed may be used.. Further, all the bladders maybe replaceable through the use of sealing members such as o-rings toprovide a good seal on a replaceable bladder.

Oversized Bladder

[0209] Referring now to FIG. 35, the shape of the flexible membrane 626in the foregoing examples is controlled in shape so as to provide amaximum resistance to movement of the patient member 207 across the axis212 when the member 207 is in contact with the membrane 626. In theprior art, membrane 626 a was attached about transducer 300 at aperiphery 654 a conforming closely to the active front surface 656 ofthe transducer 300. During operation, the membrane 626 a extends adistance D along axis 212 by an amount intended to bridge the gapbetween the transducers 300 and a patient member (not shown in FIG. 35)for different sized patient members while providing ample clearance forinsertion and removing of the patient member.

[0210] The present invention recognizes that superior patientimmobilization is obtained by expanding the point of peripheralattachment of the membrane 626 b to an outer periphery 654 bsubstantially greater than the ultimate extension distance D andpreferably greater than twice D. Generally, it is desired that thesurface of the membrane 626 extending the distance D while retaining agenerally hemispherical shape such as is believed to provide greatestresistance to cross axis motion. Recognizing that the membrane 626 maytake on other than a circular periphery, the diameter of the peripheryas used herein will refer to the diameter of a circle circumscribedwithin the periphery 654 b.

[0211] Referring now to FIG. 36, the advantage of the expanded periphery654 b may be seen by imagining the displacement of a point 658 a on themembrane 626 to a new location 658 b displaced across axis 212 such asmight be caused by motion of the patient member. Such motion wouldrequire a substantially greater stretching of the membrane 626(b) withrespect to the periphery 654 b as shown by displacement distance 660than it would require of the membrane 626(a) with respect to theperiphery 654 a as shown by displacement 662. A greater displacementfrom these points corresponds to a greater stretching of the curvedmembrane 626(b), thus a greater restoring force provided by the membrane626(b) in preventing that movement. Motion of the patient member alongaxis 212 may be controlled by controlling the pressure within themembrane 626 to provide the desired degree of immobilization.

Inflated Bladder with Fixed Bladder Support

[0212] Referring now to FIG. 37, the benefits of a closed environmentfor the acoustic coupling fluid 640 may also be obtained in aninflation-type system wherein cylindrical chambers 664 having open endsfacing each other along axis 212 with front opposed surfaces enclosed bymembranes 626 and rear surfaces connected by means of hydraulic tubing666 to pump chamber 668. The chambers 664 are fixed in separation andwith respect to the transducers contained therein. The transducers maybe any combination of arrays 300 and/or individual transducers 121 ashave been previously described. Valves 641 communicating with theinterior of the chambers 664 may allow removal of coupling fluid 640 orthe bleeding out of entrapped air. An open face of the pump chamber 668may be closed by a rolling diaphragm 670 being essentially a flexiblemembrane attached at its edges to close the chamber 558 and attached atits center to a piston 672. Displacement of the piston 672 in toward thechamber 668 creates a predefined reduction in volume of the chamber 668causing, through hydraulic equalization, a distention of membranes 626outward toward the patient member contained between them. The rollingdiaphragm 670 requires no sliding seals, as are found in conventionalpiston pumps, nor creates the possibility of backwash as can occur withperistaltic-type pumps. Further, the flow rate provided to the chambers664 need not be measured in order to determine the amount of distentionof the membranes 626 as there will be a fixed and easily calculatedrelationship between movement of the piston 672 and distention of themembranes 626. This relationship is not as easily calculated with pumpsrequiring check valves or having backwash or leakage. By hermeticallyclosing the coupling fluid 640 from outside environments, the couplingfluid may be degassed and contamination may be essentially eliminated.

[0213] The transducer chambers 664, interconnecting hydraulic tubing,and 666 pump chamber 668 all may be partially formed from orincorporated into a thermally conductive matrix such as an aluminumblock to form an effective a single heating chamber whose temperaturemay be controlled by activation of resistance heating elements 164 alsoin thermal communication with the heating chamber 132 and monitored bythermistor 168 as previously disclosed. This arrangement eliminates theneed for separate thermal regulation circuitry for each of theseelements and provides good uniformity of temperature of the variouselements.

[0214] The chamber 558 may be in communication with a pressure sensorand the piston 672 further controlled to monitor the pressure in thechamber 558 to substantially 1 PSIG. As will be described below, thissame mechanism may be used to pre-inflate the membranes 626 prior toinsertion of the patient's foot. In this case, the pressure sensor maybe used to monitor the pressure as a function of volume of liquid in thechambers 664, and hence inflation of the membranes 626, to determinethat the patient's foot is not in place during the inflation process.Presence of the patient's foot would increase the pressure sensed by thepressure sensor for a given inflation volume and thus may also be usedto confirm that the patient's foot has been inserted.

Pre-Inflated Bladder System

[0215] Referring now to FIG. 38, an ultrasonic densitometer 710 includesa housing 712 having an aperture 714 and its upper surface exposing areceptacle 718 sized to receive a human heel therein when the housing712 placed on the floor in front of a seated patient.

[0216] Attached to left and right walls 716 of a receptacle 718 andpositioned below the aperture 714 are left and right bladders 720presenting opposed convex bladder surfaces 722 formed of distendedmembranes. It will be recognized that a similar system may be used witha single bladder in a reflection mode where one of the bladders 720 isreplaced simply by a soft material providing support for the side of theheel. Further, the bladders 720 may be used not with single transducersbut with transducer arrays or a combination of single transducers orsingle elements of array transducers and arrays may be used.

[0217] Referring also to FIG. 39, prior to insertion of the patient'sheel 724, the bladders 720 are pre-inflated so that surfaces 722 definetherebetween a cavity sized to be smaller than a standard human heel724. The bladders 720 are pre-inflated with a coupling liquid, typicallywater 726, to distend a sheet of silicone rubber to a nearlyhemispherical configuration as described above. While the bladders 720are pre-inflated before the heel 724 is inserted, they are initiallyinflated by a pump system such as described above and may be deflatedfor storage or shipping. The act of inflation is, of course, intrinsicto any inflated bladder.

[0218] The water is contained between each bladder surface 722 and acorresponding backer plate 728 is attached to the membrane of thebladder 720 at its circular periphery. Positioned within the cavitydefined by the backer plate 728 and the bladder surface 722 is anultrasonic transducer 730 as described above. The transducers 730 andbacker plates 728 are held by structure 721 in fixed separation andessentially fixed with respect to the foot plate 738 as will bedescribed.

[0219] The ultrasonic transducers 730 are aligned along an ultrasonicpropagation axis 732 extending therebetween and intersecting the centerof the bladder surfaces 722. A thin coating of ultrasonic coupling gel734 is placed on the outer surface of the membrane forming the bladdersurface 722 to provide lubrication and coupling for the later insertionof a heel 724.

[0220] Referring now to FIGS. 39 and 40, the heel 724 may be inserted ina downward direction 736 across (i.e., perpendicular) to the ultrasonicpropagation axis 732 so as to slide pass past the bladder surfaces 722deforming them inward, the heel 724 to abut a heel plate 738 stoppingfurther downward motion of the heel 724. The elastic nature of themembranes of bladder surfaces 722 causes the surfaces 722 to deform asthey slide along the outer surfaces of the heel 724 in a wiping actionreducing entrapment of air between the bladder surfaces 722 and the heel724. The hemispherical shape of the bladder surfaces 722 and thematerial of their construction allows them to conform to the shape ofthe heel 724 and remain substantially aligned with the ultrasonictransducers 730 during the insertion of the heel in contrast to otherbladder shapes which as a result of their longitudinal extension orconical shape may collapse or fold over with the insertion of a theheel.

[0221] Referring now to FIG. 41, the heel plate 738 may generally have alower surface 740 supporting the sole of the foot and back surface 742perpendicular to surface 740 supporting the back of the heel 724.Surfaces 740 and 742 may be generally perpendicular to each other with740 tipped upward toward the toe end with respect to the upper surfaceof the housing 712. The toe end of surface 740 may rest to pivot about alower support fulcrum 744 while the upper edge of back surface 742 maybe engaged by a stationary load cell 746 fixed with respect to thehousing 712.

[0222] The load cell 746 may measure downward pressure 747 of the heel724 on the plate 738 and backward pressure 749 of the heel 724 on thesurface 742 so as to insure proper seating of the heel 724 against theplate 738 and proper location of the ultrasonic propagation axis 732 inthe desired region of the os calcis 748.

[0223] Referring now to FIG. 42, the downward force 747 (F1) andbackward force 749 (F2) may be monitored by the microprocessor 38(described above) so as to ensure that the forces (F1 and F2) of a givenmeasurement lie within a desired range 750 indicating proper seating ofthe foot against the plate 738 without undue force thereon andindicating further that the calf of a patient's leg abutting a calfsupport 120 (shown in FIG. 38 in phantom) is not holding the back of theheel 724 away from surface 742.

[0224] In an alternate embodiment, not shown, the bladders 720 may bepre-inflated so as to touch and in this way provide an acoustic pathdirectly from one ultrasonic transducer 730 to the other without air gapor intervening heel 724. This provides a separation distance of zero.This configuration may be used to provide a pre-measurement standardpulse between ultrasonic transducers 734 as has been previouslydescribed, and/or to provide a positive indication (by measurement ofthe ultrasonic propagation) as to whether the patient's heel 724 hasbeen inserted between the bladders or not. The shape and composition ofthe bladders 720 is such as to allow the foot to slide between thebladders 720 even when there is initially no appreciable gap between thebladders 724.

[0225] It will be within the understanding of one of ordinary skill inthe art, that the features of the various embodiments described hereinmay be interchanged with other embodiments to effect the purposesdescribed herein and therefore that the inventor contemplates theconstruction of commercial devices including either combinations offeatures of several embodiments disclosed herein or less than all thefeatures of any one embodiment. It is specifically intended that thepresent invention not be specifically limited to the embodiments andillustrations contained herein, but embrace all such modified formsthereof as come within the scope of the following claims.

We claim:
 1. An ultrasonic densitometer comprising: a pre-inflatedacoustic coupling system presenting bladder surfaces opposed along anultrasonic propagation axis and, prior to insertion of a human heel,separated by a distance smaller than the width of a human heel; acoupling material contained within the bladder surfaces; a firstultrasonic transducer communicating with the coupling material andpositioned to direct ultrasonic signals through the coupling materialalong the ultrasonic propagation axis; circuitry for excitation of thefirst ultrasonic transducer to measure propagation of ultrasonicsignals; wherein the bladder surfaces are shaped to remain aligned withthe propagation axis and in sliding contract with a human heel, when thehuman heel is inserted between the bladder surfaces across theultrasonic propagation axis.
 2. The ultrasonic densitometer of claim 1wherein the bladder surfaces are substantially hemispherical.
 3. Theultrasonic densitometer of claim 1 wherein the first and second bladdersurfaces are attached to a fixed support surface and wherein theultrasonic transducer is fixed with respect to the fixed supportsurface.
 4. The ultrasonic densitometer of claim 1 including further asecond ultrasonic transducer opposed to the first ultrasonic transducerhaving fixed separation with respect to the first ultrasonic transducer.5. The ultrasonic densitometer of claim 1 further including a pump forpre-inflating the bladder system with the coupling material prior touse.
 6. The ultrasonic densitometer of claim 1 further including a valvefor releasing material from within the bladder system.
 7. The ultrasonicdensitometer of claim 1 wherein the bladders surfaces are comprised ofan elastomeric membrane having a surface coating of ultrasonic couplinggel.
 8. The ultrasonic densitometer of claim 1 including a foot supportpositioned beneath the opposed bladder surfaces with respect to adirection of insertion of the human heel, wherein the foot supportincludes at least one sensor indicating seating of a human foot againstthe foot support.
 9. The ultrasonic densitometer of claim 8 wherein thesensor is a force sensor indicating a force selected from the groupconsisting of downward force normal to the sole of a human foot seatedagainst the foot support and backward force parallel the sole of a humanfoot seated against the foot support.
 10. The ultrasonic densitometer ofclaim 1 wherein the first ultrasonic transducer is selected from thegroup consisting of: a single ultrasonic transducer element and an arrayof ultrasonic transducer elements.
 11. The ultrasonic densitometer ofclaim 1 including a second ultrasonic transducer opposed to the firstultrasonic transducer, wherein the second ultrasonic transducer isselected from the group consisting of: a single ultrasonic transducerelement and an array of ultrasonic transducer elements.
 12. Anultrasonic densitometer comprising: a presence sensor providing a signalindicating the presence of a patient's foot at a measurement location;at least one bladder positioned with respect to the measurement locationto present a bladder surface adjacent to a patient's foot located in themeasurement location; a pump communicating with the bladder to inflatethe bladder with a coupling material contained behind the bladdersurface; circuitry communicating with the pump and the sensing means toproduce an error condition when the pump inflates the bladder while afoot is positioned measurement location; a first ultrasonic transducercoupled to the coupling material and positioned to direct ultrasonicsignals through the coupling material toward the measurement locationalong the ultrasonic propagation axis; circuitry for excitation of thefirst ultrasonic transducer for measurement of propagation of ultrasonicsignals; whereby the patient's foot must be slid across the pre-inflatedbladder to avoid an error condition.
 13. The ultrasonic densitometer ofclaim 12 wherein the error condition is a display to the operator. 14.The ultrasonic densitometer of claim 12 wherein the sensing means is aforce sensor communicating with a foot support to sense the pressure ofthe patient's foot against the foot support.
 15. The ultrasonicdensitometer of claim 12 wherein sensing means is a pressure sensorcommunicating with the bladder to measure excess pressure therein. 16.The ultrasonic densitometer of claim 12 wherein the first ultrasonictransducer is selected from the group consisting of: a single ultrasonictransducer element and an array of ultrasonic transducer elements. 17.The ultrasonic densitometer of claim 12 including a second ultrasonictransducer opposed to the first ultrasonic transducer, wherein thesecond ultrasonic transducer is selected from the group consisting of: asingle ultrasonic transducer element and an array of ultrasonictransducer elements.
 18. A method of making ultrasonic measurements of ahuman heel comprising the steps of: (a) supporting a first and secondbladder surface to define therebetween a distance smaller than the humanheel, the first and second bladder surfaces containing a couplingmaterial; (b) directing at least one ultrasonic transducer through thecoupling material along an ultrasonic propagation axis between the firstand second bladder surfaces; (c) inserting a human heel across thepropagation axis between the first and second bladder surfaces causingdeformation of the bladder surfaces to enlarge the cavity to accept thehuman heel; and (d) exciting the ultrasonic transducer to perform ameasurement of the human heel.
 19. The method claim 18 including thestep of applying an ultrasonic coupling gel to the bladder surfacesprior to insertion of the foot at step (c).
 20. The method of claim 18including the step of pre-inflating the first and second bladdersurfaces with coupling material prior to step (a).
 21. The method ofclaim 20 including the step of sensing the presence of a human heelbetween the first and second bladder surfaces prior to pre-inflation ofthe bladders and when a human heel is in place in the cavity prior topre-inflation creating an error condition.
 22. The method of claim 20including the step of releasing coupling material from within the firstand second bladder surfaces after step (d).
 23. An ultrasonicdensitometer comprising: a patient support; an ultrasonic transducersystem including at least one ultrasonic transducer positioned withrespect to the patient support to direct ultrasonic signals through apatient positioned by the patient support and to receive the ultrasonicsignals after attenuation by the patient; a coupling volume positionedbetween the ultrasonic transducer and the patient, the coupling volumealternatively occupied by a first coupling material having a first knownsound propagation property and a second coupling material having asecond known sound propagation property different from the first soundpropagation property; and a computer communicating with the ultrasonictransducer system to measure a first ultrasound propagation through boththe patient and the first coupling material and a second ultrasonicpropagation through both the patient and the second coupling material,the computer executing a stored program to compare the first and secondultrasonic propagations to produce an indication of bone quality of bonewithin the patient.
 24. The ultrasonic densitometer of claim 23 whereinthe coupling volume holds a first and second overlapping flexiblebladder alternately filled with liquid first and second couplingmaterials.
 25. The ultrasonic densitometer of claim 23 wherein the firstand second coupling materials have different sound attenuations.
 26. Theultrasonic densitometer of claim 23 wherein the first and secondcoupling materials have different sound propagation speeds.
 27. Anultrasonic densitometer for measuring bone in vivo comprising: anultrasonic transducer system including at least one ultrasonictransducer positioned with respect to the bone to direct ultrasonicsignals through the bone and to receive the ultrasonic signals afterattenuation by the bone; a computer communicating with the ultrasonictransducer system to: (i) measure a series of ultrasound signals throughthe bone as a function of time; (ii) fit the measured series to apredetermined asymptotic curve; and (iii) output a measurement based onthe asymptotic curve.
 28. The ultrasonic densitometer of claim 27 hereinthe output is based on a portion of the asymptotic curve beyond themeasured series.
 29. The ultrasonic densitometer of claim 27 wherein thetransducer array system includes two transducers opposed about the boneand wherein the ultrasonic transducer system alternates the direction ofpropagation of ultrasound through the bone between the two transducers.30. The ultrasonic densitometer of claim 27 wherein the measure of theseries of ultrasound signals is a measure of a speed of ultrasonicpropagation.
 31. The ultrasonic densitometer of claim 27 wherein themeasure of the series of ultrasound signals is a measure of ultrasonicattenuation.
 32. The ultrasonic densitometer of claim 27 wherein themeasure of the series of ultrasound signals is a combination of measuresof speed of ultrasonic propagation and ultrasonic attenuation.